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

Source/core/dom/custom/CustomElementUpgradeCandidateMap.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:
  *
  *     * 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 94 matching lines @@
 
     for (ElementSet::const_iterator candidate = candidates->begin(); candidate != candidates->end(); ++candidate) {
         unobserve(*candidate);
         m_upgradeCandidates.remove(*candidate);
     }
     return candidates.release();
 }
 
 void CustomElementUpgradeCandidateMap::trace(Visitor* visitor)
 {
+#if ENABLE(OILPAN)
     visitor->trace(m_upgradeCandidates);
     visitor->trace(m_unresolvedDefinitions);
+#endif
     CustomElementObserver::trace(visitor);
 }
 
 }