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

Source/core/xml/XMLHttpRequestProgressEventThrottle.cpp

 /*
  * Copyright (C) 2010 Julien Chaffraix <jchaffraix@webkit.org>  All right reserved.
  * Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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 203 matching lines @@
     // could insert new active DOM objects to the list.
     // m_deferEvents is kept true until all deferred events have been dispatched.
     m_dispatchDeferredEventsTimer.startOneShot(0, FROM_HERE);
 }
 
 void XMLHttpRequestProgressEventThrottle::trace(Visitor* visitor)
 {
     visitor->trace(m_target);
     visitor->trace(m_deferredProgressEvent);
     visitor->trace(m_deferredEvents);
+    TimerBase::trace(visitor);
 }
 
 } // namespace blink