Add metrics for the number of attached displays.

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"
 #include "chrome/browser/chrome_browser_main_extra_parts.h"
+#include "ui/gfx/display_observer.h"
 
 class ChromeBrowserMainParts;
 
 namespace chrome {
 void AddMetricsExtraParts(ChromeBrowserMainParts* main_parts);
 }
 
-class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
+class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts,
+                                           public gfx::DisplayObserver {
  public:
   ChromeBrowserMainExtraPartsMetrics();
   ~ChromeBrowserMainExtraPartsMetrics() override;
 
   // Overridden from ChromeBrowserMainExtraParts:
   void PreProfileInit() override;
   void PreBrowserStart() override;
   void PostBrowserStart() override;
 
  private:
 #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();
+
+  // A cached value for the number of displays.
+  int display_count_;

[Ilya Sherman, 2014/10/29 01:30:10]

nit: Please leave a blank line after this one.

[erikchen, 2014/10/29 17:15:41]

Done.

+  // This parameter is set to true after an instance of this class makes itself

[Ilya Sherman, 2014/10/29 01:30:10]

nit: "parameter" -> "variable" (though, I'd actually suggest shortening this
comment to something like "True iff |this| instance is registered as an observer
of the native screen.")

[erikchen, 2014/10/29 17:15:41]

Went with your suggestion.

+  // an observer of the native screen.
+  bool is_screen_observer_;
+
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetrics);
 };
 
 #endif  // CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_H_