[Media Router] Add UMA metrics for Media Router Dialog WebUI performance.

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 45bee3333154048d339a4b4b54c3960ec96260d4..557ae032056e28936bff3f19d3a9e9d3bb1db9ee 100644
--- a/chrome/browser/resources/media_router/media_router_ui_interface.js
+++ b/chrome/browser/resources/media_router/media_router_ui_interface.js
@@ -69,6 +69,7 @@ cr.define('media_router.ui', function() {
     container.routeList = data['routes'];
     container.initializeCastModes(data['castModes'],
         data['initialCastModeType']);
+    media_router.browserApi.onInitialDataReceived();
   }
 
   /**
@@ -144,6 +145,13 @@ cr.define('media_router.browserApi', function() {
   }
 
   /**
+   * Indicates that the initial data has been received.
+   */
+  function onInitialDataReceived() {
+    chrome.send('onInitialDataReceived');
+  }
+
+  /**
    * Reports the current number of sinks.
    *
    * @param {number} sinkCount
@@ -176,6 +184,7 @@ cr.define('media_router.browserApi', function() {
     actOnIssue: actOnIssue,
     closeDialog: closeDialog,
     closeRoute: closeRoute,
+    onInitialDataReceived: onInitialDataReceived,
     reportSinkCount: reportSinkCount,
     requestInitialData: requestInitialData,
     requestRoute: requestRoute,