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

--HG--
extra : moz-landing-system : lando

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;
   }
 

dom/media/webvtt/vtt.jsm

@@ -1021,6 +1021,10 @@ ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
         controlBar = controls.parentNode.getElementById("controlBar");
       }
       controlBarShown = controlBar ? !!controlBar.clientHeight : false;
+    } else {
+      // There is no controls element. This only happen to UA Widget because
+      // it is created lazily.
+      controlBarShown = false;
     }
 
     // Determine if we need to compute the display states of the cues. This could