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

Source/core/frame/BarProp.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
@@ skipping 32 matching lines @@
     public:
         enum Type { Locationbar, Menubar, Personalbar, Scrollbars, Statusbar, Toolbar };
 
         static PassRefPtrWillBeRawPtr<BarProp> create(LocalFrame* frame, Type type)
         {
             return adoptRefWillBeNoop(new BarProp(frame, type));
         }
 
         bool visible() const;
 
-        void trace(Visitor*) { }
+        virtual void trace(Visitor*) { }
 
     private:
         BarProp(LocalFrame*, Type);
         Type m_type;
     };
 
 } // namespace blink
 
 #endif // BarProp_h