Add metrics for the number of attached displays.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics_mac.mm

+// 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.
+
+#include "chrome/browser/metrics/chrome_browser_main_extra_parts_metrics_mac.h"
+
+#include "base/metrics/histogram.h"
+#include "ui/gfx/screen.h"
+
+ChromeBrowserMainExtraPartsMetricsMac::ChromeBrowserMainExtraPartsMetricsMac() {
+  display_count_ = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  UMA_HISTOGRAM_COUNTS_100("OSX.DisplayCount.StartUp", display_count_);
+  gfx::Screen::GetNativeScreen()->AddObserver(this);
+}
+
+ChromeBrowserMainExtraPartsMetricsMac::
+    ~ChromeBrowserMainExtraPartsMetricsMac() {
+  gfx::Screen::GetNativeScreen()->RemoveObserver(this);
+}
+
+void ChromeBrowserMainExtraPartsMetricsMac::OnDisplayAdded(
+    const gfx::Display& new_display) {
+  EmitDisplaysChangedMetric();
+}
+
+void ChromeBrowserMainExtraPartsMetricsMac::OnDisplayRemoved(
+    const gfx::Display& old_display) {
+  EmitDisplaysChangedMetric();
+}
+
+void ChromeBrowserMainExtraPartsMetricsMac::OnDisplayMetricsChanged(
+    const gfx::Display& display,
+    uint32_t changed_metrics) {
+}
+
+void ChromeBrowserMainExtraPartsMetricsMac::EmitDisplaysChangedMetric() {
+  int display_count = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  if (display_count != display_count_) {
+    display_count_ = display_count;
+    UMA_HISTOGRAM_COUNTS_100("OSX.DisplayCount.Changed", display_count_);
+  }
+}