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"
+
+// This class records OSX metrics at start up, and during the lifetime of the
+// application.
+class ChromeBrowserMainExtraPartsMetricsMac {
+ public:
+  ChromeBrowserMainExtraPartsMetricsMac();
+  ~ChromeBrowserMainExtraPartsMetricsMac();
+
+ private:
+  // A pointer to the ObjC bridge used to listen to NSNotificationCenter
+  // notifications. Casted to a void* so that this file can be included in C++
+  // translation units.
+  void* metrics_bridge_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChromeBrowserMainExtraPartsMetricsMac);
+};
+
+#endif  // CHROME_BROWSER_METRICS_CHROME_BROWSER_MAIN_EXTRA_PARTS_METRICS_MAC_H_