[Media Router] Allow casting new media to sink with existing route.

chrome/browser/resources/media_router/elements/route_details/route_details.js

 // 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.
 
 // This Polymer element shows information from media that is currently cast
 // to a device.
 Polymer({
   is: 'route-details',
 
   properties: {
@@ skipping 42 matching lines @@
    * Fires a close-route-click event. This is called when the button to close
    * the current route is clicked.
    *
    * @private
    */
   closeRoute_: function() {
     this.fire('close-route-click', {route: this.route});
   },
 
   /**
-   * Fires a start-casting-to-route-click event. This is called when the button
-   * to start casting to the current route is clicked.
+   * Fires a start-casting-to-route-click event if the current route is
+   * joinable, otherwise it fires a cast-new-media-click event. This is called
+   * when the button to start casting to the current route is clicked.
    *
    * @private
    */
   startCastingToRoute_: function() {
-    this.fire('start-casting-to-route-click', {route: this.route});
+    if (this.route.canJoin) {
+      this.fire('start-casting-to-route-click', {route: this.route});
+    } else {
+      this.fire('cast-new-media-click', {route: this.route});
+    }
   },
 
   /**
    * Loads the custom controller if |route.customControllerPath| exists.
    * Falls back to the default route details view otherwise, or if load fails.
    * Updates |activityStatus_| for the default view.
    *
    * @private
    */
   maybeLoadCustomController_: function() {
@@ skipping 20 matching lines @@
     extensionview.load(this.route.customControllerPath)
     .then(function() {
       // Load was successful; show the custom controller.
       that.isCustomControllerHidden_ = false;
     }, function() {
       // Load was unsuccessful; fall back to default view.
       that.isCustomControllerHidden_ = true;
     });
   },
 });