Add startup metrics that measure the performance of the first web contents.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h

Index: chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
diff --git a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
index 91b904d77375f5a13d43d063a1b58a60ae874ed0..618cb51a22af42f8ec67a6c5c7aefb08a68b984e 100644
--- a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
+++ b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
@@ -29,6 +29,8 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts,
   void PostBrowserStart() override;
 
  private:
+  class FirstWebContentsProfiler;
+
 #if defined(OS_MACOSX) && !defined(OS_IOS)
   // Records Mac specific metrics.
   void RecordMacMetrics();
@@ -43,6 +45,9 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts,
   // If the number of displays has changed, emit a UMA metric.
   void EmitDisplaysChangedMetric();
 
+  // Called by |first_web_contents_profiler_| when it wants to be destroyed.
+  void FirstWebContentsProfilerWantsDestruction();

[Ilya Sherman, 2014/11/26 02:24:55]

This feels pretty hacky.  I wonder if it would be cleaner to just have the
FirstWebContentsProfiler own its own memory, i.e. call "delete this" when it
currently calls FirstWebContentsProfilerWantsDestruction().

[erikchen, 2014/11/26 03:48:43]

I don't like the style where objects own their own memory, as I find it prone to
memory leaks. The logic is the same, but I changed the code to use the
delegation pattern, which I assume will be okay by you in terms of not feeling
hacky.

+
   // A cached value for the number of displays.
   int display_count_;
 
@@ -50,6 +55,9 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts,
   // screen.
   bool is_screen_observer_;
 
+  // Measures start up performance of the first active web contents.
+  scoped_ptr<FirstWebContentsProfiler> first_web_contents_profiler_;
+
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetrics);
 };