Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

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 43 matching lines @@
 
     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*) { }
 
 private:
     explicit History(LocalFrame*);
 
     KURL urlForState(const String& url);
 
     SerializedScriptValue* stateInternal() const;
 
     RefPtr<SerializedScriptValue> m_lastStateObjectRequested;
 };
 
 } // namespace blink
 
 #endif // History_h