Add a pure virtual method to the GarbageCollectedMixin class

Source/core/dom/DocumentParserClient.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DocumentParserClient_h
 #define DocumentParserClient_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class DocumentParserClient : public WillBeGarbageCollectedMixin {
 public:
     // This callback is called when all data pushed to parser has been consumed.
     virtual void notifyParserStopped() = 0;
 
+    virtual void trace(Visitor*) { }
+
 protected:
     DocumentParserClient() { }
 };
 
 } // namespace blink
 
 #endif // DocumentParserClient_h