Add metrics for the number of attached displays.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics_mac.h

+// Copyright 2014 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_MAC_H_
+#define CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_MAC_H_
+
+#include "base/macros.h"
+#include "ui/gfx/display_observer.h"
+
+// This class records OSX metrics at start up, and during the lifetime of the
+// application.
+class ChromeBrowserMainExtraPartsMetricsMac : public gfx::DisplayObserver {
+ public:
+  ChromeBrowserMainExtraPartsMetricsMac();
+  virtual ~ChromeBrowserMainExtraPartsMetricsMac() override;

[Robert Sesek, 2014/10/09 14:42:28]

Is |override| right here?

[erikchen, 2014/10/09 22:29:27]

Yes. http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Inheritance
"""Explicitly annotate overrides of virtual functions or virtual destructors
with an override or (less frequently) final specifier."""

The style guide indicates that the keywords |virtual| and |override| should not
both be used, but the Chrome style checker isn't ready for that yet.

+
+ 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 displayCount_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetricsMac);
+};
+
+#endif  // CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_MAC_H_