Add MediaSessionObserver to allow clients observe MediaSession directly

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index bc2bfbb066280408bc461f74678ed938f1d40d80..0633f7025283ac461ebe472a9cee50d9c427c4ed 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -60,7 +60,7 @@
 #include "content/browser/media/audio_stream_monitor.h"
 #include "content/browser/media/capture/web_contents_audio_muter.h"
 #include "content/browser/media/media_web_contents_observer.h"
-#include "content/browser/media/session/media_session.h"
+#include "content/browser/media/session/media_session_impl.h"
 #include "content/browser/message_port_message_filter.h"
 #include "content/browser/plugin_content_origin_whitelist.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"
@@ -3787,7 +3787,7 @@ void WebContentsImpl::OnUpdateFaviconURL(
 }
 
 void WebContentsImpl::OnMediaSessionStateChanged() {
-  MediaSession* session = MediaSession::Get(this);
+  MediaSessionImpl* session = MediaSessionImpl::Get(this);
   for (auto& observer : observers_) {
     observer.MediaSessionStateChanged(session->IsControllable(),
                                        session->IsSuspended());
@@ -3795,22 +3795,22 @@ void WebContentsImpl::OnMediaSessionStateChanged() {
 }
 
 void WebContentsImpl::OnMediaSessionMetadataChanged() {
-  MediaSession* session = MediaSession::Get(this);
+  MediaSessionImpl* session = MediaSessionImpl::Get(this);
   for (auto& observer : observers_) {
     observer.MediaSessionMetadataChanged(session->metadata());
   }
 }
 
 void WebContentsImpl::ResumeMediaSession() {
-  MediaSession::Get(this)->Resume(MediaSession::SuspendType::UI);
+  MediaSessionImpl::Get(this)->Resume(MediaSession::SuspendType::UI);
 }
 
 void WebContentsImpl::SuspendMediaSession() {
-  MediaSession::Get(this)->Suspend(MediaSession::SuspendType::UI);
+  MediaSessionImpl::Get(this)->Suspend(MediaSession::SuspendType::UI);
 }
 
 void WebContentsImpl::StopMediaSession() {
-  MediaSession::Get(this)->Stop(MediaSession::SuspendType::UI);
+  MediaSessionImpl::Get(this)->Stop(MediaSession::SuspendType::UI);
 }
 
 void WebContentsImpl::OnPasswordInputShownOnHttp() {