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

Source/modules/mediastream/MediaDevicesRequest.h

 /*
  * Copyright (C) 2014 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:
  * 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 36 matching lines @@
     MediaDeviceInfoCallback* callback() const { return m_callback.get(); }
     Document* ownerDocument();
 
     void start();
 
     void succeed(const MediaDeviceInfoVector&);
 
     // ActiveDOMObject
     virtual void stop() OVERRIDE;
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 private:
     MediaDevicesRequest(ExecutionContext*, UserMediaController*, PassOwnPtr<MediaDeviceInfoCallback>);
 
     UserMediaController* m_controller;
 
     OwnPtr<MediaDeviceInfoCallback> m_callback;
 };
 
 } // namespace blink
 
 #endif // MediaDevicesRequest_h