Add MediaSessionObserver to allow clients observe MediaSession directly

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* session) {
+  MediaSessionImpl* session_impl = static_cast<MediaSessionImpl*>(session);
+  if (session_impl)
+    session_impl->AddObserver(this);
+}
+
+}  // namespace content