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

chrome/browser/media/router/media_router_mojo_impl_unittest.cc

Index: chrome/browser/media/router/media_router_mojo_impl_unittest.cc
diff --git a/chrome/browser/media/router/media_router_mojo_impl_unittest.cc b/chrome/browser/media/router/media_router_mojo_impl_unittest.cc
index 09e2572a905779199d7bcc6be16031709c99426e..870d881657fb2148dfd6ec84f62a995143453726 100644
--- a/chrome/browser/media/router/media_router_mojo_impl_unittest.cc
+++ b/chrome/browser/media/router/media_router_mojo_impl_unittest.cc
@@ -273,10 +273,21 @@ TEST_F(MediaRouterMojoImplTest, RegisterAndUnregisterMediaRoutesObserver) {
   ProcessEventLoop();
 }
 
+void AssertPostMessageResult(bool expected, bool actual) {
+  EXPECT_EQ(expected, actual);
+}
+
 TEST_F(MediaRouterMojoImplTest, PostMessage) {
   EXPECT_CALL(mock_mojo_media_router_service_,
-              PostMessage(mojo::String(kRouteId), mojo::String(kMessage)));
-  router()->PostMessage(kRouteId, kMessage);
+              PostMessage(mojo::String(kRouteId), mojo::String(kMessage), _))
+      .WillOnce(
+          Invoke([](const MediaRoute::Id& route_id, const std::string& message,
+                    const interfaces::MediaRouter::PostMessageCallback& cb) {
+            cb.Run(true);
+          }));
+
+  router()->PostMessage(kRouteId, kMessage,
+                        base::Bind(&AssertPostMessageResult, true));
   ProcessEventLoop();
 }