[Chromecast] Fix media session blocking tests.

content/public/browser/media_session_observer.cc

 // Copyright 2016 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.
 
 #include "content/public/browser/media_session_observer.h"
 
 #include "content/browser/media/session/media_session_impl.h"
 
 namespace content {
 
 MediaSessionObserver::MediaSessionObserver(MediaSession* media_session)
-    : media_session_(static_cast<MediaSessionImpl*>(media_session)) {
+    : media_session_(media_session) {
   if (media_session_)
     media_session_->AddObserver(this);
 }
 
 MediaSessionObserver::~MediaSessionObserver() {
   StopObserving();
 }
 
 MediaSession* MediaSessionObserver::media_session() const {
   return media_session_;
 }
 
 void MediaSessionObserver::StopObserving() {
   if (media_session_)
     media_session_->RemoveObserver(this);
   media_session_ = nullptr;
 }
 
 }  // namespace content