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

Source/core/workers/WorkerLocation.h

 /*
  * Copyright (C) 2008 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 34 matching lines @@
         return adoptRefWillBeNoop(new WorkerLocation(url));
     }
 
     virtual KURL url() const OVERRIDE { return m_url; }
     virtual String input() const OVERRIDE
     {
         ASSERT_NOT_REACHED();
         return String();
     }
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 private:
     explicit WorkerLocation(const KURL& url) : m_url(url)
     {
         ScriptWrappable::init(this);
     }
 
     KURL m_url;
 };
 
 } // namespace blink
 
 #endif // WorkerLocation_h