Add metrics for the number of attached displays.

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 4ac4c06f6b9f930c790d2a5859d679023139deed..7b588ddd4662da2755a9ca91bed6a2f217bbe7de 100644
--- a/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
+++ b/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h
@@ -9,6 +9,7 @@
 #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;
 
@@ -16,7 +17,8 @@ namespace chrome {
 void AddMetricsExtraParts(ChromeBrowserMainParts* main_parts);
 }
 
-class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
+class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts,
+                                           public gfx::DisplayObserver {
  public:
   ChromeBrowserMainExtraPartsMetrics();
   ~ChromeBrowserMainExtraPartsMetrics() override;
@@ -32,6 +34,21 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
   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);
 };