Make VTTParserClient the GC mixin it needs to be.

third_party/WebKit/Source/core/html/track/vtt/VTTParser.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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 543 matching lines @@
         break;
     }
     default:
         break;
     }
 }
 
 DEFINE_TRACE(VTTParser)
 {
     visitor->trace(m_document);
+    visitor->trace(m_client);
     visitor->trace(m_cueList);
     visitor->trace(m_regionList);
 }
 
 } // namespace blink