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

content/browser/media/session/media_session.cc

Index: content/browser/media/session/media_session.cc
diff --git a/content/browser/media/session/media_session.cc b/content/browser/media/session/media_session.cc
index f4d3c0979e38a34cf3e57e399cd5c81f9797b5c6..45160327151240618797606f6efb463f474e4108 100644
--- a/content/browser/media/session/media_session.cc
+++ b/content/browser/media/session/media_session.cc
@@ -5,6 +5,7 @@
 #include "content/browser/media/session/media_session.h"
 
 #include "content/browser/media/session/audio_focus_delegate.h"
+#include "content/browser/media/session/media_session_observer.h"
 #include "content/browser/media/session/media_session_player_observer.h"
 #include "content/browser/web_contents/web_contents_impl.h"
 #include "content/public/browser/web_contents.h"
@@ -59,6 +60,23 @@ MediaSession::~MediaSession() {
   DCHECK(audio_focus_state_ == State::INACTIVE);
 }
 
+void MediaSession::AddObserver(MediaSessionObserver* observer) {
+  LOG(INFO) << "MediaSession::AddObserver()";
+  observers_.AddObserver(observer);
+}
+
+void MediaSession::RemoveObserver(MediaSessionObserver* observer) {
+  LOG(INFO) << "MediaSession::RemoveObserver()";
+  observers_.RemoveObserver(observer);
+  owned_observers_.erase(observer);
+}
+
+void MediaSession::PassObserverOwnership(std::unique_ptr<MediaSessionObserver> observer) {
+  LOG(INFO) << "MediaSession::PassObserverOwnership()";
+  DCHECK(!owned_observers_.count(observer.get()));
+  owned_observers_[observer.get()] = std::move(observer);
+}
+
 void MediaSession::WebContentsDestroyed() {
   // This should only work for tests. In production, all the players should have
   // already been removed before WebContents is destroyed.
@@ -74,8 +92,9 @@ void MediaSession::WebContentsDestroyed() {
 
 void MediaSession::SetMetadata(const base::Optional<MediaMetadata>& metadata) {
   metadata_ = metadata;
-  static_cast<WebContentsImpl*>(web_contents())
-      ->OnMediaSessionMetadataChanged();
+
+  for (auto& observer : observers_)
+    observer.MediaSessionMetadataChanged(metadata_);
 }
 
 bool MediaSession::AddPlayer(MediaSessionPlayerObserver* observer,
@@ -403,8 +422,10 @@ void MediaSession::AbandonSystemAudioFocusIfNeeded() {
 }
 
 void MediaSession::UpdateWebContents() {
+  // TODO in this CL: make media_session_state_listeners inherit from MediaSessionObserver.
   media_session_state_listeners_.Notify(audio_focus_state_);
-  static_cast<WebContentsImpl*>(web_contents())->OnMediaSessionStateChanged();
+  for (auto& observer : observers_)
+    observer.MediaSessionStateChanged(IsControllable(), IsSuspended());
 }
 
 void MediaSession::SetAudioFocusState(State audio_focus_state) {