Oilpan: fix build after r186944.

Source/core/inspector/InspectorDebuggerAgent.cpp

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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.
@@ skipping 1486 matching lines @@
         m_frontend->globalObjectCleared();
 }
 
 void InspectorDebuggerAgent::trace(Visitor* visitor)
 {
     visitor->trace(m_injectedScriptManager);
     visitor->trace(m_listener);
     visitor->trace(m_asyncCallStackTracker);
 #if ENABLE(OILPAN)
     visitor->trace(m_promiseTracker);
+    visitor->trace(m_asyncOperationsForStepInto);
 #endif
     InspectorBaseAgent::trace(visitor);
 }
 
 } // namespace blink