Make VTTParserClient the GC mixin it needs to be.

third_party/WebKit/Source/core/html/track/vtt/VTTParser.h

 /*
  * 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 29 matching lines @@
 #include "core/html/track/vtt/VTTTokenizer.h"
 #include "platform/heap/Handle.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/text/StringBuilder.h"
 
 namespace blink {
 
 class Document;
 class VTTScanner;
 
-class VTTParserClient {
+class VTTParserClient : public GarbageCollectedMixin {
 public:
     virtual ~VTTParserClient() { }
 
     virtual void newCuesParsed() = 0;
     virtual void newRegionsParsed() = 0;
     virtual void fileFailedToParse() = 0;
+
+    DEFINE_INLINE_VIRTUAL_TRACE() { }
 };
 
 class VTTParser final : public GarbageCollectedFinalized<VTTParser> {
 public:
     enum ParseState {
         Initial,
         Header,
         Id,
         TimingsAndSettings,
         CueText,
@@ skipping 70 matching lines @@
     static bool collectTimeStamp(VTTScanner& input, double& timeStamp);
 
     BufferedLineReader m_lineReader;
     OwnPtr<TextResourceDecoder> m_decoder;
     AtomicString m_currentId;
     double m_currentStartTime;
     double m_currentEndTime;
     StringBuilder m_currentContent;
     String m_currentSettings;
 
-    VTTParserClient* m_client;
+    Member<VTTParserClient> m_client;
 
     HeapVector<Member<TextTrackCue>> m_cueList;
 
     HeapVector<Member<VTTRegion>> m_regionList;
 };
 
 } // namespace blink
 
 #endif