Oilpan: trace non-owned InspectorOverlay references.

Source/core/inspector/InspectorPageAgent.cpp

 /*
  * Copyright (C) 2011 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 1357 matching lines @@
 {
     m_overlay->setPausedInDebuggerMessage(message);
 }
 
 DEFINE_TRACE(InspectorPageAgent)
 {
     visitor->trace(m_page);
     visitor->trace(m_injectedScriptManager);
     visitor->trace(m_debuggerAgent);
     visitor->trace(m_cssAgent);
+    visitor->trace(m_overlay);
     visitor->trace(m_inspectorResourceContentLoader);
     InspectorBaseAgent::trace(visitor);
 }
 
 } // namespace blink
-