[Media Router] Add P1 UMA metrics for Media Router.

chrome/browser/media/router/media_router_metrics.h

+// Copyright 2015 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_MEDIA_ROUTER_MEDIA_ROUTER_METRICS_H_
+#define CHROME_BROWSER_MEDIA_ROUTER_MEDIA_ROUTER_METRICS_H_
+
+namespace media_router {
+
+// Where the user clicked to open the Media Router dialog.
+enum MediaRouterDialogOpenOrigin {
+  // NOTE: Do not renumber these as that would confuse interpretation of
+  // previously logged data. When making changes, also update the enum list
+  // in tools/metrics/histograms/histograms.xml to keep it in sync.
+  TOOLBAR = 0,
+  OVERFLOW_MENU = 1,
+  CONTEXTUAL_MENU = 2,
+  PAGE = 3,
+
+  // NOTE: Add entries only immediately above this line.
+  TOTAL_COUNT = 4
+};
+
+class MediaRouterMetrics {
+ public:
+  // Records where the user clicked to open the Media Router dialog.
+  static void RecordMediaRouterDialogOrigin(
+      MediaRouterDialogOpenOrigin origin);
+};
+
+}  // namespace media_router
+
+#endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_ROUTER_METRICS_H_