Oilpan: trace non-owned InspectorOverlay references.

Source/core/inspector/InspectorDOMAgent.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * 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
@@ skipping 2338 matching lines @@
         return errorString->isEmpty();
     }
     return true;
 }
 
 DEFINE_TRACE(InspectorDOMAgent)
 {
     visitor->trace(m_domListener);
     visitor->trace(m_pageAgent);
     visitor->trace(m_injectedScriptManager);
+    visitor->trace(m_overlay);
 #if ENABLE(OILPAN)
     visitor->trace(m_documentNodeToIdMap);
     visitor->trace(m_danglingNodeToIdMaps);
     visitor->trace(m_idToNode);
     visitor->trace(m_idToNodesMap);
     visitor->trace(m_document);
     visitor->trace(m_revalidateTask);
     visitor->trace(m_searchResults);
 #endif
     visitor->trace(m_history);
     visitor->trace(m_domEditor);
     visitor->trace(m_listener);
     InspectorBaseAgent::trace(visitor);
 }
 
 } // namespace blink