[Memory] Remove LowMemoryPressure and flags to disable memory discard…

…ing.

Since LowMemoryPressure is not being anymore, this removes it, which reduces the Chromeos-specific code.

Note: This is the second step of expanding Chromeos tab killing to other platforms.

BUG=463597

Review URL: https://codereview.chromium.org/1201453002

Cr-Commit-Position: refs/heads/master@{#335602}

chrome/app/chromeos_strings.grdp

@@ -1214,12 +1214,6 @@ Press any key to continue exploring.
   <message name="IDS_FLAGS_DISABLE_DISPLAY_COLOR_CALIBRATION_DESCRIPTION" desc="Description for the flag to disable the color calibration of the display.">
     Disable calibrating the color of the display even if the display supports the feature.
   </message>
-  <message name="IDS_FLAGS_DISABLE_MEMORY_PRESSURE_NAME" desc="Name for the flag to disable memory pressure handling on ChromeOS.">
-    Disable memory pressure handling
-  </message>
-  <message name="IDS_FLAGS_DISABLE_MEMORY_PRESSURE_DESCRIPTION" desc="Description for the flag to disable memory pressure handling on ChromeOS.">
-    Disable enhanced memory pressure handling on ChromeOS.
-  </message>
   <message name="IDS_FLAGS_MEMORY_PRESSURE_THRESHOLD_NAME" desc="Name for the flag which specifies which memory pressure strategy should be used on ChromeOS.">
     Memory discard strategy for advanced pressure handling
   </message>

chrome/browser/about_flags.cc

@@ -1706,12 +1706,6 @@ const Experiment kExperiments[] = {
      SINGLE_VALUE_TYPE(switches::kEnableMaterialDesignSettings)},
 #endif
 #if defined(OS_CHROMEOS)
-    {"disable-memory-pressure-chromeos",
-     IDS_FLAGS_DISABLE_MEMORY_PRESSURE_NAME,
-     IDS_FLAGS_DISABLE_MEMORY_PRESSURE_DESCRIPTION,
-     kOsCrOS,
-     SINGLE_VALUE_TYPE(
-         chromeos::switches::kDisableMemoryPressureSystemChromeOS)},
     {"memory-pressure-thresholds",
      IDS_FLAGS_MEMORY_PRESSURE_THRESHOLD_NAME,
      IDS_FLAGS_MEMORY_PRESSURE_THRESHOLD_DESCRIPTION,

chrome/browser/memory/oom_priority_manager.h

@@ -25,8 +25,6 @@ class GURL;
 
 namespace memory {
 
-class LowMemoryObserver;
-
 // The OomPriorityManager periodically checks (see
 // ADJUSTMENT_INTERVAL_SECONDS in the source) the status of renderers
 // and adjusts the out of memory (OOM) adjustment value (in
@@ -160,12 +158,6 @@ class OomPriorityManager : public content::NotificationObserver {
   // Holds the focused tab's child process host id.
   ProcessInfo focused_tab_process_info_;
 
-  // The old observer for the kernel low memory signal. This is null if
-  // the new MemoryPressureListener is used.
-  // TODO(skuhne): Remove this when the enhanced memory observer is turned on
-  // by default.
-  scoped_ptr<LowMemoryObserver> low_memory_observer_;
-
   // A listener to global memory pressure events. This will be used if the
   // memory pressure system was instantiated - otherwise the LowMemoryObserver
   // will be used.

chrome/browser/memory/oom_priority_manager_chromeos.cc

@@ -27,7 +27,6 @@
 #include "build/build_config.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/browser_process_platform_part_chromeos.h"
-#include "chrome/browser/memory/low_memory_observer_chromeos.h"
 #include "chrome/browser/memory/oom_memory_details.h"
 #include "chrome/browser/memory/system_memory_stats_recorder.h"
 #include "chrome/browser/ui/browser.h"
@@ -104,10 +103,6 @@ OomPriorityManager::OomPriorityManager()
     : focused_tab_process_info_(std::make_pair(0, 0)),
       discard_count_(0),
       recent_tab_discard_(false) {
-  // Use the old |LowMemoryObserver| when there is no |MemoryPressureMonitor|.
-  if (!base::MemoryPressureMonitor::Get())
-    low_memory_observer_.reset(new LowMemoryObserver);
-
   registrar_.Add(this, content::NOTIFICATION_RENDERER_PROCESS_CLOSED,
                  content::NotificationService::AllBrowserContextsAndSources());
   registrar_.Add(this, content::NOTIFICATION_RENDERER_PROCESS_TERMINATED,
@@ -131,33 +126,23 @@ void OomPriorityManager::Start() {
         this, &OomPriorityManager::RecordRecentTabDiscard);
   }
   start_time_ = TimeTicks::Now();
-  // If a |LowMemoryObserver| exists we use the old system, otherwise we create
-  // a |MemoryPressureListener| to listen for memory events.
-  if (low_memory_observer_) {
-    low_memory_observer_->Start();
-  } else {
-    base::MemoryPressureMonitor* monitor = base::MemoryPressureMonitor::Get();
-    if (monitor) {
-      memory_pressure_listener_.reset(
-          new base::MemoryPressureListener(base::Bind(
-              &OomPriorityManager::OnMemoryPressure, base::Unretained(this))));
-      base::MemoryPressureListener::MemoryPressureLevel level =
-          monitor->GetCurrentPressureLevel();
-      if (level ==
-          base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL) {
-        OnMemoryPressure(level);
-      }
+  // Create a |MemoryPressureListener| to listen for memory events.
+  base::MemoryPressureMonitor* monitor = base::MemoryPressureMonitor::Get();
+  if (monitor) {
+    memory_pressure_listener_.reset(new base::MemoryPressureListener(base::Bind(
+        &OomPriorityManager::OnMemoryPressure, base::Unretained(this))));
+    base::MemoryPressureListener::MemoryPressureLevel level =
+        monitor->GetCurrentPressureLevel();
+    if (level == base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL) {
+      OnMemoryPressure(level);
     }
   }
 }
 
 void OomPriorityManager::Stop() {
   timer_.Stop();
   recent_tab_discard_timer_.Stop();
-  if (low_memory_observer_)
-    low_memory_observer_->Stop();
-  else
-    memory_pressure_listener_.reset();
+  memory_pressure_listener_.reset();
 }
 
 std::vector<base::string16> OomPriorityManager::GetTabTitles() {
@@ -407,21 +392,19 @@ void OomPriorityManager::Observe(int type,
       content::RenderProcessHost* host =
           content::Source<content::RenderProcessHost>(source).ptr();
       oom_score_map_.erase(host->GetID());
-      if (!low_memory_observer_) {
-        // Coming here we know that a renderer was just killed and memory should
-        // come back into the pool. However - the memory pressure observer did
-        // not yet update its status and therefore we ask it to redo the
-        // measurement, calling us again if we have to release more.
-        // Note: We do not only accelerate the discarding speed by doing another
-        // check in short succession - we also accelerate it because the timer
-        // driven MemoryPressureMonitor will continue to produce timed events
-        // on top. So the longer the cleanup phase takes, the more tabs will
-        // get discarded in parallel.
-        base::chromeos::MemoryPressureMonitor* monitor =
-            base::chromeos::MemoryPressureMonitor::Get();
-        if (monitor)
-          monitor->ScheduleEarlyCheck();
-      }
+      // Coming here we know that a renderer was just killed and memory should
+      // come back into the pool. However - the memory pressure observer did
+      // not yet update its status and therefore we ask it to redo the
+      // measurement, calling us again if we have to release more.
+      // Note: We do not only accelerate the discarding speed by doing another
+      // check in short succession - we also accelerate it because the timer
+      // driven MemoryPressureMonitor will continue to produce timed events
+      // on top. So the longer the cleanup phase takes, the more tabs will
+      // get discarded in parallel.
+      base::chromeos::MemoryPressureMonitor* monitor =
+          base::chromeos::MemoryPressureMonitor::Get();
+      if (monitor)
+        monitor->ScheduleEarlyCheck();
       break;
     }
     case content::NOTIFICATION_RENDER_WIDGET_VISIBILITY_CHANGED: {

chrome/chrome_browser.gypi

@@ -625,8 +625,6 @@
       'browser/memory_details_linux.cc',
       'browser/memory_details_mac.cc',
       'browser/memory_details_win.cc',
-      'browser/memory/low_memory_observer_chromeos.cc',
-      'browser/memory/low_memory_observer_chromeos.h',
       'browser/memory/oom_memory_details_chromeos.cc',
       'browser/memory/oom_memory_details.h',
       'browser/memory/oom_priority_manager_chromeos.cc',