Oilpan: Fix tracing of events through XMLHttpRequestProgressEventThrottle.

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 198 matching lines @@
         return;
     }
 
     // Do not dispatch events inline here, since ExecutionContext is iterating over
     // the list of active DOM objects to resume them, and any activated JS event-handler
     // 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_deferredProgressEvent);
+    visitor->trace(m_deferredEvents);
+}
+
 } // namespace WebCore