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

chrome/browser/resources/media_router/media_router.js

Index: chrome/browser/resources/media_router/media_router.js
diff --git a/chrome/browser/resources/media_router/media_router.js b/chrome/browser/resources/media_router/media_router.js
index 4773a16d3101867158506cc563fcfb569eca4667..a566c7dfba5ba7eba69ba42a949a9e40600eaac6 100644
--- a/chrome/browser/resources/media_router/media_router.js
+++ b/chrome/browser/resources/media_router/media_router.js
@@ -26,6 +26,7 @@ cr.define('media_router', function() {
     container.addEventListener('close-route-click', onCloseRouteClick);
     container.addEventListener('create-route', onCreateRoute);
     container.addEventListener('issue-action-click', onIssueActionClick);
+    container.addEventListener('report-sink-count', onSinkCountReported);
   }
 
   /**
@@ -80,6 +81,18 @@ cr.define('media_router', function() {
     media_router.browserApi.closeRoute(data.detail.route);
   }
 
+  /**
+   * Reports the current sink count.
+   * Called 3 seconds after the dialog is initially opened.
+   *
+   * @param {{detail: {sinkCount: number}}} data
+   * Parameters in |data|.detail:
+   *   sinkCount - the number of sinks.
+   */
+  function onSinkCountReported(data) {
+    media_router.browserApi.reportSinkCount(data.detail.sinkCount);
+  }
+
   return {
     initialize: initialize,
   };