Oilpan: Remove support for tracing off-heap HashSets.

Source/core/css/StyleSheetContents.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 667 matching lines @@
         if (!m_importRules[i]->styleSheet())
             continue;
         m_importRules[i]->styleSheet()->findFontFaceRules(fontFaceRules);
     }
 
     findFontFaceRulesFromRules(childRules(), fontFaceRules);
 }
 
 void StyleSheetContents::trace(Visitor* visitor)
 {
+#if ENABLE(OILPAN)
     visitor->trace(m_ownerRule);
     visitor->trace(m_importRules);
     visitor->trace(m_childRules);
     visitor->trace(m_loadingClients);
     visitor->trace(m_completedClients);
     visitor->trace(m_ruleSet);
+#endif
 }
 
 }