[MediaRouter] Update MR-2-Extension's PostMessage to return boolean.

chrome/browser/media/router/media_router.mojom

Index: chrome/browser/media/router/media_router.mojom
diff --git a/chrome/browser/media/router/media_router.mojom b/chrome/browser/media/router/media_router.mojom
index e67b327f5571e0419906741eeae7448f81d9ac6e..12fefa9570136be4fe22b702048466058c4c0737 100644
--- a/chrome/browser/media/router/media_router.mojom
+++ b/chrome/browser/media/router/media_router.mojom
@@ -96,7 +96,8 @@ interface MediaRouter {
 
   // Sends |message| with optional |extra_info_json| via the media route
   // |media_route_id|.
-  PostMessage(string media_route_id, string message);
+  // If the operation was sucessfull, |sent| is true; otherwise it is false.
+  PostMessage(string media_route_id, string message) => (bool sent);
 
   // Starts querying for sinks capable of displaying |media_source|.
   StartObservingMediaSinks(string media_source);