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

Source/core/streams/Stream.h

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 63 matching lines @@
     void neuter() { m_isNeutered = true; }
     bool isNeutered() const { return m_isNeutered; }
 
     // Implementation of ActiveDOMObject.
     //
     // FIXME: Implement suspend() and resume() when necessary.
     virtual void suspend() OVERRIDE;
     virtual void resume() OVERRIDE;
     virtual void stop() OVERRIDE;
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 protected:
     Stream(ExecutionContext*, const String& mediaType);
 
     // This is an internal URL referring to the blob data associated with this object. It serves
     // as an identifier for this blob. The internal URL is never used to source the blob's content
     // into an HTML or for FileRead'ing, public blob URLs must be used for those purposes.
     KURL m_internalURL;
 
     String m_mediaType;
 
     bool m_isNeutered;
 };
 
 } // namespace blink
 
 #endif // Stream_h