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

chrome/browser/resources/media_router/media_router_ui_interface.js

Index: chrome/browser/resources/media_router/media_router_ui_interface.js
diff --git a/chrome/browser/resources/media_router/media_router_ui_interface.js b/chrome/browser/resources/media_router/media_router_ui_interface.js
index 74061595b4c86a2f3d5d02b4e35441d5ea9e816a..45bee3333154048d339a4b4b54c3960ec96260d4 100644
--- a/chrome/browser/resources/media_router/media_router_ui_interface.js
+++ b/chrome/browser/resources/media_router/media_router_ui_interface.js
@@ -144,6 +144,15 @@ cr.define('media_router.browserApi', function() {
   }
 
   /**
+   * Reports the current number of sinks.
+   *
+   * @param {number} sinkCount
+   */
+  function reportSinkCount(sinkCount) {
+    chrome.send('reportSinkCount', [sinkCount]);
+  }
+
+  /**
    * Requests data to initialize the WebUI with.
    * The data will be returned via media_router.ui.setInitialData.
    */
@@ -167,6 +176,7 @@ cr.define('media_router.browserApi', function() {
     actOnIssue: actOnIssue,
     closeDialog: closeDialog,
     closeRoute: closeRoute,
+    reportSinkCount: reportSinkCount,
     requestInitialData: requestInitialData,
     requestRoute: requestRoute,
   };