Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/css/StyleSheetList.cpp

 /**
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007 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 37 matching lines @@
 }
 
 void StyleSheetList::detachFromDocument()
 {
     // FIXME: Oilpan: This is safe currently because the Document is
     // *not* in the oilpan heap and the style engine is alive when the
     // document dies. When the Document is in the oilpan heap, we need
     // the StyleSheetList and the document to die together and get rid
     // of the detachedStyleSheets.
     m_detachedStyleSheets = document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
-    m_treeScope = 0;
+    m_treeScope = nullptr;
 }
 
 unsigned StyleSheetList::length()
 {
     return styleSheets().size();
 }
 
 StyleSheet* StyleSheetList::item(unsigned index)
 {
     const WillBeHeapVector<RefPtrWillBeMember<StyleSheet> >& sheets = styleSheets();
@@ skipping 18 matching lines @@
 CSSStyleSheet* StyleSheetList::anonymousNamedGetter(const AtomicString& name)
 {
     HTMLStyleElement* item = getNamedItem(name);
     if (!item)
         return 0;
     return item->sheet();
 }
 
 void StyleSheetList::trace(Visitor* visitor)
 {
+    visitor->trace(m_treeScope);
     visitor->trace(m_detachedStyleSheets);
 }
 
 } // namespace WebCore