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

Source/platform/mhtml/ArchiveResourceCollection.cpp

 /*
  * Copyright (C) 2008 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:
  *
  * 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 82 matching lines @@
 {
     RefPtrWillBeRawPtr<MHTMLArchive> archive = m_subframes.take(frameName);
     if (archive)
         return archive.release();
 
     return m_subframes.take(url.string());
 }
 
 void ArchiveResourceCollection::trace(Visitor* visitor)
 {
-#if ENABLE(OILPAN)
     visitor->trace(m_subresources);
     visitor->trace(m_subframes);
-#endif
 }
 
 }