[Media Router WebUI] Prevent error when route becomes null

Once the route-details element is stamped, getSinkForRoute_ is bound to
currentRoute_. This becomes null if the route is stopped which causes
getSinkForRoute_ to throw an innocuous but noisy error. This change adds
a check of the route before dereferencing it.

BUG=614144
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2053743002
Cr-Commit-Position: refs/heads/master@{#399281}

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

@@ -1178,11 +1178,11 @@ Polymer({
 
   /**
    * @param {?media_router.Route} route Route to get the sink for.
-   * @return {?media_router.Sink|undefined} Sink associated with |route| or
+   * @return {?media_router.Sink} Sink associated with |route| or
    *     undefined if we don't have data for the sink.
    */
   getSinkForRoute_: function(route) {
-    return this.sinkMap_[route.sinkId];
+    return route ? this.sinkMap_[route.sinkId] : null;
   },
 
   /**