Let CastMediaBlocker observe MediaSession instead of WebContents [FOR TRY]

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)
+    : session_(static_cast<MediaSessionImpl*>(session)) {
+  if (session_)
+    session_->AddObserver(this);
+}
+
+MediaSessionObserver::~MediaSessionObserver() {
+  if (session_)
+    session_->RemoveObserver(this);
+}
+
+MediaSession* MediaSessionObserver::media_session() const {
+  return session_;
+}
+
+void MediaSessionObserver::StopObserving() {
+  if (session_)
+    session_->RemoveObserver(this);
+  session_ = nullptr;
+}
+
+}  // namespace content