Oilpan: Remove support for tracing off-heap HashSets.

Source/core/animation/AnimationTimeline.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 228 matching lines @@
 #if !ENABLE(OILPAN)
 void AnimationTimeline::detachFromDocument()
 {
     // FIXME: AnimationTimeline should keep Document alive.
     m_document = nullptr;
 }
 #endif
 
 void AnimationTimeline::trace(Visitor* visitor)
 {
+#if ENABLE(OILPAN)
     visitor->trace(m_document);
     visitor->trace(m_timing);
     visitor->trace(m_playersNeedingUpdate);
     visitor->trace(m_players);
+#endif
 }
 
 } // namespace