[oilpan]: Remove support for tracing off-heap hashmaps.

Source/core/dom/custom/CustomElementScheduler.cpp

 /*
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 140 matching lines @@
         CustomElementCallbackDispatcher::instance().enqueue(&callbackQueue);
         return callbackQueue;
     }
 
     CustomElementMicrotaskDispatcher::instance().enqueue(&callbackQueue);
     return callbackQueue;
 }
 
 void CustomElementScheduler::trace(Visitor* visitor)
 {
+#if ENABLE(OILPAN)
     visitor->trace(m_elementCallbackQueueMap);
+#endif
 }
 
 } // namespace blink