Link MediaSessionTabHelper with native MediaSession [CL is going to be split]

content/browser/media/session/media_session_observer.h

+// 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.
+
+#ifndef CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_OBSERVER_H_
+#define CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_OBSERVER_H_
+
+#include "base/macros.h"
+#include "base/optional.h"
+#include "content/common/content_export.h"
+#include "content/public/common/media_metadata.h"
+
+namespace content {
+
+class MediaSession;
+
+// The observer for MediaSession events.
+class CONTENT_EXPORT MediaSessionObserver {
+ public:
+  // Create a MediaSessionObserver and start observing |session|.
+  explicit MediaSessionObserver(MediaSession* session);
+  // Destructor of MediaSessionObserver. It will remove itself from |session| if
+  // the observer is still observing a MediaSession.
+  virtual ~MediaSessionObserver();
+
+  // Gets the observed MediaSession.
+  MediaSession* session() { return session_; }
+
+  // Called when the observed MediaSession is destroyed.
+  virtual void MediaSessionDestroyed() {}
+
+  // Called when the observed MediaSession has changed its state.
+  virtual void MediaSessionStateChanged(bool is_controllable,
+                                        bool is_suspended) {}
+
+  // Called when the observed MediaSession has changed metadata.
+  virtual void MediaSessionMetadataChanged(
+      const base::Optional<MediaMetadata>& metadata) {}
+
+ private:
+  // Weak pointer to MediaSession.
+  MediaSession* session_;
+
+  DISALLOW_COPY_AND_ASSIGN(MediaSessionObserver);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_OBSERVER_H_