Bug 1491066 - Allow TextTrackManager to pass cue without videocontrols r=alwu

With UA Widget, the videocontrols container is created lazily.
It won't be a problem for WebVTT.processCues() in vtt.jsm, so
TextTrackManager::UpdateCueDisplay() should not early return there, but pass
nullptr to it.

Differential Revision: https://phabricator.services.mozilla.com/D3667

dom/html/TextTrackManager.cpp

@@ -276,7 +276,7 @@ TextTrackManager::UpdateCueDisplay()
 
   nsCOMPtr<nsIContent> overlay = videoFrame->GetCaptionOverlay();
   nsCOMPtr<nsIContent> controls = videoFrame->GetVideoControls();
-  if (!overlay || !controls) {
+  if (!overlay) {
     return;
   }