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"
+
+namespace {
+
+// The maximum number of screens to record in UMA.
+int kMaxDisplays = 10;
+
+}  // namespace
+
+ChromeBrowserMainExtraPartsMetricsMac::ChromeBrowserMainExtraPartsMetricsMac() {
+  displayCount_ = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  UMA_HISTOGRAM_ENUMERATION("OSX.DisplayCount.StartUp",
+                            std::min(kMaxDisplays, displayCount_),
+                            kMaxDisplays + 1);
+  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 displayCount = gfx::Screen::GetNativeScreen()->GetNumDisplays();
+  if (displayCount != displayCount_) {
+    displayCount_ = displayCount;
+    UMA_HISTOGRAM_ENUMERATION("OSX.DisplayCount.Changed",
+                              std::min(kMaxDisplays, displayCount_),
+                              kMaxDisplays + 1);
+  }
+}