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

content/browser/media/session/media_session.h

Index: content/browser/media/session/media_session.h
diff --git a/content/browser/media/session/media_session.h b/content/browser/media/session/media_session.h
index 9239c7854a18b18326e2c2983451e49490922982..ae0b5a2b429005877bc33108266eddf57ccad062 100644
--- a/content/browser/media/session/media_session.h
+++ b/content/browser/media/session/media_session.h
@@ -10,6 +10,7 @@
 #include "base/callback_list.h"
 #include "base/id_map.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "base/optional.h"
 #include "content/browser/media/session/audio_focus_manager.h"
 #include "content/browser/media/session/media_session_uma_helper.h"
@@ -29,6 +30,7 @@ namespace content {
 class AudioFocusDelegate;
 class AudioFocusManagerTest;
 class MediaSessionPlayerObserver;
+class MediaSessionObserver;
 class MediaSessionStateObserver;
 class MediaSessionVisibilityBrowserTest;
 
@@ -71,6 +73,17 @@ class MediaSession : public WebContentsObserver,
 
   ~MediaSession() override;
 
+  // Add and remove an observer for MediaSession messages. Clients must remove
+  // the observer before they go away, or pass the observer ownership to
+  // MediaSession via PassObserverOwnership (then the observer will go away when
+  // the session is destroyed.
+  void AddObserver(MediaSessionObserver* observer);
+  void RemoveObserver(MediaSessionObserver* observer);
+
+  // Pass a MediaSessionObserver to be owned and managed by this session. The
+  // observer will be removed when the session is destroyed.
+  void PassObserverOwnership(std::unique_ptr<MediaSessionObserver> observer);
+
   void SetMetadata(const base::Optional<MediaMetadata>& metadata);
   const base::Optional<MediaMetadata>& metadata() const { return metadata_; }
 
@@ -237,6 +250,13 @@ class MediaSession : public WebContentsObserver,
   base::Optional<MediaMetadata> metadata_;
   base::CallbackList<void(State)> media_session_state_listeners_;
 
+  // The list of observers notified when the MediaSession changes.
+  base::ObserverList<MediaSessionObserver> observers_;
+
+  // The MediaSessionObservers that are owned by this session, they will be
+  // destroyed and removed.
+  std::vector<std::unique_ptr<MediaSessionObserver>> owned_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(MediaSession);
 };