Add documentation in memory_benchmarks.md on finding links to trace f…

…iles

This CL:
- incorporates the feedback (see linked bug) that
  memory_benchmarks.md doesn't mention where to find the trace files on
  perf dashboard.
- adds
  "memory:chrome:all_processes:reported_by_chrome:net:effective_size_avg"
  string in a comment in url_request_context.h, so it's greppable in
  code and makes it easier to find where the metric comes from.

BUG=734619

Review-Url: https://codereview.chromium.org/2952203002
Cr-Commit-Position: refs/heads/master@{#481871}

docs/memory-infra/memory_benchmarks.md

@@ -92,6 +92,12 @@ To view data from one of the benchmarks on the
 *   **Subtest (3):** The name of a *[user story](#User-stories)*
     (with `:` replaced by `_`).
 
+If you are investigating a Perf dashboard alert and would like to see the
+details, you can click on any point of the graph. It gives you the commit range,
+buildbot output and a link to the trace file taken during the buildbot run.
+
+![Chrome Performance Dashboard Alert](https://storage.googleapis.com/chromium-docs.appspot.com/perfdashboard_alert.png)
+
 ## How to run the benchmarks
 
 Benchmarks may be run on a local platform/device or remotely on a try job.
@@ -170,7 +176,13 @@ where:
 *   **source:** One of `reported_by_chrome` or `reported_by_os`
 *   **component:** May be a Chrome component, e.g. `skia` or `sqlite`;
     details about a specific component, e.g. `v8:heap`; or a class of memory
-    as seen by the OS, e.g. `system_memory:native_heap` or `gpu_memory`.
+    as seen by the OS, e.g. `system_memory:native_heap` or `gpu_memory`. If
+    reported by chrome, the metrics are gathered by `MemoryDumpProvider`s,
+    probes placed in the specific components' codebase. For example, in
+    "memory:chrome:all_processes:reported_by_chrome:net:effective_size_avg,"
+    the component is "net" which is Chrome's network stack and
+    "reported_by_chrome" means that this metric is gathered via probes in
+    the network stack.
 *   **kind:** The kind of memory being reported. For metrics reported by
     Chrome this usually is `effective_size` (others are `locked_size`
     and `allocated_objects_size`); for metrics by the OS this usually is

net/url_request/url_request_context.h

@@ -281,6 +281,8 @@ class NET_EXPORT URLRequestContext
   void set_name(const char* name) { name_ = name; }
 
   // MemoryDumpProvider implementation:
+  // This is reported as
+  // "memory:chrome:all_processes:reported_by_chrome:net:effective_size_avg."
   bool OnMemoryDump(const base::trace_event::MemoryDumpArgs& args,
                     base::trace_event::ProcessMemoryDump* pmd) override;