Bug 1601385 - Don't continue UpdateOutputTrackSources if the tracks-captured state changed. r=jib

Changing the tracks-captured state will trigger another run of
UpdateOutputTrackSources, so continuing the current one is futile. The next run
will properly re-evaluate all input state to this function.

Differential Revision: https://phabricator.services.mozilla.com/D100004

dom/html/HTMLMediaElement.cpp

@@ -3686,6 +3686,9 @@ void HTMLMediaElement::UpdateOutputTrackSources() {
     mOutputStreams.RemoveElementAt(i);
     if (mOutputStreams.IsEmpty()) {
       mTracksCaptured = nullptr;
+      // mTracksCaptured is one of the Watchables triggering this method.
+      // Unsetting it here means we'll run through this method again very soon.
+      return;
     }
   }