Convert /chrome/browser/ash/crosapi to ScopedDisplayObserver

This is a refactoring to make it easier for future changes to
DisplayObserver, and to clean up manual Add/Remove calls with RAII.

ScopedDisplayObserver is an RAII class that directly replaces
display::Screen::Add/RemoveObserver.

ScopedOptionalDisplayObserver is the same thing, but handles the case
where display::Screen::GetScreen() is null.  This can happen in tests.

This is a non-functional change, intending to replicate existing
behavior exactly.

This CL was uploaded by git cl split.

R=erikchen@chromium.org

Bug: 1225463
Change-Id: I469ca2770f91d1eb4e2927120ea8c3ab865b9e5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3004096
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898412}

chrome/browser/ash/crosapi/system_display_ash.cc

@@ -9,7 +9,6 @@
 #include "ash/public/ash_interfaces.h"
 #include "base/bind.h"
 #include "chrome/browser/extensions/system_display/system_display_serialization.h"
-#include "ui/display/screen.h"
 #include "ui/gfx/geometry/insets.h"
 #include "ui/gfx/geometry/mojom/geometry.mojom.h"
 #include "ui/gfx/geometry/rect.h"
@@ -91,13 +90,7 @@ void SystemDisplayAsh::OnDisplayConfigChanged() {
 
 void SystemDisplayAsh::StartDisplayChangedEventSources() {
   // Start Source 1.
-  if (!is_observing_screen_) {
-    display::Screen* screen = display::Screen::GetScreen();
-    if (screen) {
-      screen->AddObserver(this);
-      is_observing_screen_ = true;
-    }
-  }
+  display_observer_.emplace(this);
 
   // Start Source 2.
   if (!is_observing_cros_display_config_) {
@@ -124,12 +117,7 @@ void SystemDisplayAsh::StopDisplayChangedEventSources() {
   }
 
   // Stop Source 1.
-  if (is_observing_screen_) {
-    is_observing_screen_ = false;
-    display::Screen* screen = display::Screen::GetScreen();
-    DCHECK(screen);
-    screen->RemoveObserver(this);
-  }
+  display_observer_.reset();
 }
 
 }  // namespace crosapi

chrome/browser/ash/crosapi/system_display_ash.h

@@ -85,7 +85,7 @@ class SystemDisplayAsh : public mojom::SystemDisplay,
   mojo::RemoteSet<mojom::DisplayChangeObserver> observers_;
 
   // Source 1 status and objects.
-  bool is_observing_screen_ = false;
+  absl::optional<display::ScopedOptionalDisplayObserver> display_observer_;
 
   // Source 2 status and objects.
   bool is_observing_cros_display_config_ = false;