[Chromecast] Fix media session blocking tests.

content/public/browser/media_session.cc

+// Copyright (c) 2017 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.h"
+
+namespace content {
+
+MediaSession::MediaSession() {}
+
+MediaSession::~MediaSession() {}
+
+void MediaSession::AddObserver(MediaSessionObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void MediaSession::RemoveObserver(MediaSessionObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
+}  // namespace content