Move StyleSheet to the oilpan heap using transition types.

Source/core/css/MediaList.h

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2008, 2009, 2010, 2012 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 74 matching lines @@
     String item(unsigned index) const;
     void deleteMedium(const String& oldMedium, ExceptionState&);
     void appendMedium(const String& newMedium, ExceptionState&);
 
     String mediaText() const { return m_mediaQueries->mediaText(); }
     void setMediaText(const String&);
 
     // Not part of CSSOM.
     CSSRule* parentRule() const { return m_parentRule; }
     CSSStyleSheet* parentStyleSheet() const { return m_parentStyleSheet; }
-    void clearParentStyleSheet() { ASSERT(m_parentStyleSheet); m_parentStyleSheet = 0; }
+    void clearParentStyleSheet() { ASSERT(m_parentStyleSheet); m_parentStyleSheet = nullptr; }
     void clearParentRule() { ASSERT(m_parentRule); m_parentRule = nullptr; }
     const MediaQuerySet* queries() const { return m_mediaQueries.get(); }
 
     void reattach(MediaQuerySet*);
 
     void trace(Visitor*);
 
 private:
     MediaList();
     MediaList(MediaQuerySet*, CSSStyleSheet* parentSheet);
     MediaList(MediaQuerySet*, CSSRule* parentRule);
 
     RefPtrWillBeMember<MediaQuerySet> m_mediaQueries;
-    // Cleared in ~CSSStyleSheet destructor.
-    CSSStyleSheet* m_parentStyleSheet;
+    // Cleared in ~CSSStyleSheet destructor when oilpan is not enabled.
+    RawPtrWillBeMember<CSSStyleSheet> m_parentStyleSheet;
     // Cleared in the ~CSSMediaRule and ~CSSImportRule destructors when oilpan is not enabled.
     RawPtrWillBeMember<CSSRule> m_parentRule;
 };
 
 // Adds message to inspector console whenever dpi or dpcm values are used for "screen" media.
 void reportMediaQueryWarningIfNeeded(Document*, const MediaQuerySet*);
 
 } // namespace
 
 #endif