[Oilpan]: Make StylePropertySet fully garbage collected.

Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013 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
@@ skipping 631 matching lines @@
 
     ElementRareData* elementRareData() const;
     ElementRareData& ensureElementRareData();
 
     void detachAllAttrNodesFromElement();
     void detachAttrNodeFromElementWithValue(Attr*, const AtomicString& value);
     void detachAttrNodeAtIndex(Attr*, size_t index);
 
     bool isJavaScriptURLAttribute(const Attribute&) const;
 
-    RefPtr<ElementData> m_elementData;
+    RefPtrWillBeMember<ElementData> m_elementData;
 };
 
 DEFINE_NODE_TYPE_CASTS(Element, isElementNode());
 template <typename T> bool isElementOfType(const Element&);
 template <typename T> inline bool isElementOfType(const Node& node) { return node.isElementNode() && isElementOfType<const T>(toElement(node)); }
 template <> inline bool isElementOfType<const Element>(const Element&) { return true; }
 
 // Type casting.
 template<typename T> inline T& toElement(Node& node)
 {
@@ skipping 213 matching lines @@
     template <> inline bool isElementOfType<const thisType>(const Element& element) { return element.predicate; } \
     DEFINE_NODE_TYPE_CASTS(thisType, predicate)
 
 #define DEFINE_ELEMENT_TYPE_CASTS_WITH_FUNCTION(thisType) \
     template <> inline bool isElementOfType<const thisType>(const Element& element) { return is##thisType(element); } \
     DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION(thisType)
 
 } // namespace
 
 #endif