Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/dom/UserActionElementSet.cpp

 /*
  * Copyright (C) 2012 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.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 98 matching lines @@
     if (result != m_elements.end()) {
         ASSERT(element->isUserActionElement());
         result->value |= flags;
         return;
     }
 
     element->setUserActionElement(true);
     m_elements.add(element, flags);
 }
 
+void UserActionElementSet::trace(Visitor* visitor)
+{
+    visitor->trace(m_elements);
 }
+
+}