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 04f3f0e1f3008fa24599288113abd9565c1a82c0..54c7cabb91afb67d15ac1ae1dd7a7eb544f7c2d3 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();
   virtual ~ChromeBrowserMainExtraPartsMetrics();
@@ -27,6 +29,18 @@ class ChromeBrowserMainExtraPartsMetrics : public ChromeBrowserMainExtraParts {
   virtual void PostBrowserStart() override;
 
  private:
+  // 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_;
+
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetrics);
 };