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 eedfcd1da8f63e2a3996c708d45a8c843e72e11a..a9dd657ca93edaaf3e949cd2ced453d948f2eeca 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"
@@ -57,6 +58,26 @@ MediaSession* MediaSession::Get(WebContents* web_contents) {
 MediaSession::~MediaSession() {
   DCHECK(players_.empty());
   DCHECK(audio_focus_state_ == State::INACTIVE);
+  owned_observers_.clear();
+}
+
+void MediaSession::AddObserver(MediaSessionObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void MediaSession::RemoveObserver(MediaSessionObserver* observer) {
+  observers_.RemoveObserver(observer);
+  for (auto iter = owned_observers_.begin(); iter != owned_observers_.end();
+       ++iter) {
+    if (iter->get() == observer)
+      owned_observers_.erase(iter);
+    return;
+  }
+}
+
+void MediaSession::PassObserverOwnership(
+    std::unique_ptr<MediaSessionObserver> observer) {
+  owned_observers_.push_back(std::move(observer));
 }
 
 void MediaSession::WebContentsDestroyed() {
@@ -74,8 +95,9 @@ 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,
@@ -408,7 +430,8 @@ void MediaSession::AbandonSystemAudioFocusIfNeeded() {
 
 void MediaSession::UpdateWebContents() {
   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) {