Added stub MediaSourcePlayer for developing behind the flag

media/base/android/media_player_android.cc

Index: media/base/android/media_player_android.cc
diff --git a/media/base/android/media_player_android.cc b/media/base/android/media_player_android.cc
index 9f224c9b675cb08e80f6cf87460cddbff65e440d..6ad9647162ff5670edafd0a6618f2c141f4ec4a8 100644
--- a/media/base/android/media_player_android.cc
+++ b/media/base/android/media_player_android.cc
@@ -81,6 +81,11 @@ void MediaPlayerAndroid::DetachListener() {
   listener_->ReleaseMediaPlayerListenerResources();
 }
 
+void MediaPlayerAndroid::DestroyListenerOnUIThread() {
+  weak_factory_.InvalidateWeakPtrs();
+  listener_.reset();
+}
+
 void MediaPlayerAndroid::SetAudible(bool is_audible) {
   if (is_audible_ != is_audible) {
     is_audible_ = is_audible;