Make VTTParserClient the GC mixin it needs to be.

third_party/WebKit/Source/core/loader/TextTrackLoader.cpp

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 150 matching lines @@
     if (m_cueParser)
         m_cueParser->getNewRegions(outputRegions);
 }
 
 DEFINE_TRACE(TextTrackLoader)
 {
     visitor->trace(m_client);
     visitor->trace(m_cueParser);
     visitor->trace(m_document);
     ResourceOwner<RawResource>::trace(visitor);
+    VTTParserClient::trace(visitor);
 }
 
 } // namespace blink