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

Source/core/dom/DocumentOrderedMap.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple 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 163 matching lines @@
     return get<keyMatchesLowercasedMapName>(key, scope);
 }
 
 Element* DocumentOrderedMap::getElementByLabelForAttribute(StringImpl* key, const TreeScope* scope) const
 {
     return get<keyMatchesLabelForAttribute>(key, scope);
 }
 
 void DocumentOrderedMap::trace(Visitor* visitor)
 {
-#if ENABLE(OILPAN)
     visitor->trace(m_map);
-#endif
 }
 
 void DocumentOrderedMap::MapEntry::trace(Visitor* visitor)
 {
     visitor->trace(element);
 #if ENABLE(OILPAN)
     visitor->trace(orderedList);
 #endif
 }
 
 } // namespace blink