Bug 1304948 - Part 2: Don't run TimeMarchesOn when shutdown. r=rillian

MozReview-Commit-ID: AE6FE74ESHN

dom/html/TextTrackManager.cpp

@@ -589,8 +589,8 @@ TextTrackManager::TimeMarchesOn()
   WEBVTT_LOG("TimeMarchesOn");
   mTimeMarchesOnDispatched = false;
 
-  // Early return if we don't have any TextTracks.
-  if (!mTextTracks || mTextTracks->Length() == 0) {
+  // Early return if we don't have any TextTracks or shutting down.
+  if (!mTextTracks || mTextTracks->Length() == 0 || mShutdown) {
     return;
   }