Change various helper classes to transition types to get CSSValue entirely onto the gc heap.

Source/core/css/CSSImportRule.h

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2006, 2008, 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 #ifndef CSSImportRule_h
 #define CSSImportRule_h
 
 #include "core/css/CSSRule.h"
+#include "heap/Handle.h"
 
 namespace WebCore {
 
 class CSSStyleSheetResource;
 class MediaList;
 class MediaQuerySet;
 class StyleRuleImport;
 
 class CSSImportRule FINAL : public CSSRule {
 public:
     static PassRefPtr<CSSImportRule> create(StyleRuleImport* rule, CSSStyleSheet* sheet) { return adoptRef(new CSSImportRule(rule, sheet)); }
 
     virtual ~CSSImportRule();
 
     virtual CSSRule::Type type() const OVERRIDE { return IMPORT_RULE; }
     virtual String cssText() const OVERRIDE;
     virtual void reattach(StyleRuleBase*) OVERRIDE;
 
     String href() const;
     MediaList* media() const;
     CSSStyleSheet* styleSheet() const;
 
 private:
     CSSImportRule(StyleRuleImport*, CSSStyleSheet*);
 
     RefPtr<StyleRuleImport> m_importRule;
-    mutable RefPtr<MediaList> m_mediaCSSOMWrapper;
+    mutable RefPtrWillBePersistent<MediaList> m_mediaCSSOMWrapper;
     mutable RefPtr<CSSStyleSheet> m_styleSheetCSSOMWrapper;
 };
 
 DEFINE_CSS_RULE_TYPE_CASTS(CSSImportRule, IMPORT_RULE);
 
 } // namespace WebCore
 
 #endif // CSSImportRule_h