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);

[whywhat, 2016/10/21 18:28:14]

what I don't like about this is that it's only used for the MediaSessionDelegate
to be owned by MediaSession. So why does it have to do with observers? What if
we keep the delegate in WebContentsAndroid?

WebContentsAndroid would create the native delegate for the WebContents's
MediaSession in its ctor (which would create the Java object in its ctor, a
common pattern) and pass its GetJavaObject to the WebContentsImpl.java like
NavigationController. 

Then in setWebContents in MediaSessionTabHelper you would just set mDelegate =
webContents.getMediaSessionDelegate() and mDelegate.setObserver(this); // or new
MediaSessionObserver() { ... };

I think that would make the codepath for creating MediaSessionDelegate and its
ownership much clearer (maybe it should be called MediaSessionProxy or
something).

+
   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);
 };