Oilpan: Remove support for tracing off-heap HashSets.

Source/core/dom/MutationObserver.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 258 matching lines @@
     for (size_t i = 0; i < observers.size(); ++i) {
         if (observers[i]->canDeliver())
             observers[i]->deliver();
         else
             suspendedMutationObservers().add(observers[i]);
     }
 }
 
 void MutationObserver::trace(Visitor* visitor)
 {
+#if ENABLE(OILPAN)
     visitor->trace(m_records);
     visitor->trace(m_registrations);
+#endif
 }
 
 } // namespace WebCore