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

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_
 #define CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 11 matching lines @@
  public:
   ChromeBrowserMainExtraPartsMetrics();
   ~ChromeBrowserMainExtraPartsMetrics() override;
 
   // Overridden from ChromeBrowserMainExtraParts:
   void PreProfileInit() override;
   void PreBrowserStart() override;
   void PostBrowserStart() override;
 
  private:
+  class FirstWebContentsProfiler;
+
 #if defined(OS_MACOSX) && !defined(OS_IOS)
   // Records Mac specific metrics.
   void RecordMacMetrics();
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
   // DisplayObserver overrides.
   virtual void OnDisplayAdded(const gfx::Display& new_display) override;
   virtual void OnDisplayRemoved(const gfx::Display& old_display) override;
   virtual void OnDisplayMetricsChanged(const gfx::Display& display,
                                        uint32_t changed_metrics) override;
 
   // 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_;
 
   // True iff |this| instance is registered as an observer of the native
   // 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);
 };
 
 #endif  // CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_