[Lockscreen Media Controls] Fix rough transition when skipping tracks.

This issue is caused by an ActionButtonVisibilityUpdate at
the end of SetIsPlaying - This will make all buttons invisible at the
beginning of a new media session when receiving
MediaSessionInfoChanged, then MediaSessionActionsChanged will update
button visibility immediately after that, which is causing the flashing
issue.

Bug: 1011160

Change-Id: I0f08fc77874641fd44025aae1c29f1cebeacdeb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838385Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704297}

ash/login/ui/lock_screen_media_controls_view.cc

@@ -530,10 +530,8 @@ void LockScreenMediaControlsView::MediaSessionChanged(
   }
 
   // If |media_session_id_| resumed while waiting, don't hide the controls.
-  if (hide_controls_timer_->IsRunning() && request_id == media_session_id_) {
+  if (hide_controls_timer_->IsRunning() && request_id == media_session_id_)
     hide_controls_timer_->Stop();
-    enabled_actions_.clear();
-  }
 
   // If this session is different than the previous one, wait to see if the
   // previous one resumes before hiding the controls.