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

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

Index: chrome/browser/resources/media_router/elements/route_details/route_details.js
diff --git a/chrome/browser/resources/media_router/elements/route_details/route_details.js b/chrome/browser/resources/media_router/elements/route_details/route_details.js
index d3db440a8042f08e4e0e3d4d2d61e075ae257cf2..261f1b121d7768d802904e8d47c96e7406723605 100644
--- a/chrome/browser/resources/media_router/elements/route_details/route_details.js
+++ b/chrome/browser/resources/media_router/elements/route_details/route_details.js
@@ -60,13 +60,18 @@ Polymer({
   },
 
   /**
-   * 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});
+    }
   },
 
   /**