Oilpan: make DOMWindowProperty a GC mixin.

Source/core/frame/History.h

 /*
  * Copyright (C) 2007 Apple 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 26 matching lines @@
 
 namespace blink {
 
 class LocalFrame;
 class KURL;
 class ExecutionContext;
 class ExceptionState;
 
 class History FINAL : public RefCountedWillBeGarbageCollectedFinalized<History>, public ScriptWrappable, public DOMWindowProperty {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(History);
 public:
     static PassRefPtrWillBeRawPtr<History> create(LocalFrame* frame)
     {
         return adoptRefWillBeNoop(new History(frame));
     }
 
     unsigned length() const;
     SerializedScriptValue* state();
 
     void back(ExecutionContext*);
     void forward(ExecutionContext*);
     void go(ExecutionContext*, int distance);
 
     bool stateChanged() const;
     bool isSameAsCurrentState(SerializedScriptValue*) const;
 
     void stateObjectAdded(PassRefPtr<SerializedScriptValue>, const String& title, const String& url, FrameLoadType, ExceptionState&);
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) OVERRIDE;
 
 private:
     explicit History(LocalFrame*);
 
     KURL urlForState(const String& url);
 
     SerializedScriptValue* stateInternal() const;
 
     RefPtr<SerializedScriptValue> m_lastStateObjectRequested;
 };
 
 } // namespace blink
 
 #endif // History_h