[Media Router] Add UMA metrics to record the load time for route controls

chrome/test/data/webui/media_router/route_details_tests.js

Index: chrome/test/data/webui/media_router/route_details_tests.js
diff --git a/chrome/test/data/webui/media_router/route_details_tests.js b/chrome/test/data/webui/media_router/route_details_tests.js
index f47a046d14a03182ae384b7cda82f899c1cf2a76..17451ad511f1fb1050c1f49da766b12d0f197f2d 100644
--- a/chrome/test/data/webui/media_router/route_details_tests.js
+++ b/chrome/test/data/webui/media_router/route_details_tests.js
@@ -18,6 +18,13 @@ cr.define('route_details', function() {
        */
       var fakeRouteOne;
 
+      /**
+       * The custom controller path for |fakeRouteOne|.
+       * @const @type {string}
+       */
+      var fakeRouteOneControllerPath =
+          'chrome-extension://123/custom_view.html';
+
       /**
        * Second fake route created before each test.
        * @type {media_router.Route}
@@ -84,9 +91,9 @@ cr.define('route_details', function() {
         document.body.appendChild(details);
 
         // Initialize routes and sinks.
-        fakeRouteOne = new media_router.Route('route id 1', 'sink id 1',
-            'Video 1', 1, true, false,
-            'chrome-extension://123/custom_view.html');
+        fakeRouteOne = new media_router.Route(
+            'route id 1', 'sink id 1', 'Video 1', 1, true, false,
+            fakeRouteOneControllerPath);
         fakeRouteTwo = new media_router.Route('route id 2', 'sink id 2',
             'Video 2', 2, false, true);
         fakeSinkOne = new media_router.Sink(
@@ -205,7 +212,9 @@ cr.define('route_details', function() {
         details.$$('extension-view-wrapper').$$('#custom-controller').load =
             function(url) {
           setTimeout(function() {
-            assertEquals('chrome-extension://123/custom_view.html', url);
+            assertEquals(
+                fakeRouteOneControllerPath,
+                url.substring(0, fakeRouteOneControllerPath.length));
             checkCustomControllerIsShown();
             done();
           });