Oilpan: Prepare to move IdTargetObserver and IdTargetObserverRegistry to Oilpan heap.

Source/core/dom/TreeScope.cpp

 /*
  * Copyright (C) 2011 Google Inc. All Rights Reserved.
  * Copyright (C) 2012 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 524 matching lines @@
         if (style && style->hasViewportUnits())
             element->setNeedsStyleRecalc(LocalStyleChange);
     }
 }
 
 void TreeScope::trace(Visitor* visitor)
 {
     visitor->trace(m_rootNode);
     visitor->trace(m_document);
     visitor->trace(m_parentTreeScope);
+    visitor->trace(m_idTargetObserverRegistry);
     visitor->trace(m_selection);
 }
 
 } // namespace WebCore