Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/StyleRuleImport.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.
  *
@@ skipping 24 matching lines @@
 
 class StyleRuleImport : public StyleRuleBase {
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED;
 public:
     static PassRefPtrWillBeRawPtr<StyleRuleImport> create(const String& href, PassRefPtr<MediaQuerySet>);
 
     ~StyleRuleImport();
 
     StyleSheetContents* parentStyleSheet() const { return m_parentStyleSheet; }
     void setParentStyleSheet(StyleSheetContents* sheet) { ASSERT(sheet); m_parentStyleSheet = sheet; }
-    void clearParentStyleSheet() { m_parentStyleSheet = 0; }
+    void clearParentStyleSheet() { m_parentStyleSheet = nullptr; }
 
     String href() const { return m_strHref; }
     StyleSheetContents* styleSheet() const { return m_styleSheet.get(); }
 
     bool isLoading() const;
     MediaQuerySet* mediaQueries() { return m_mediaQueries.get(); }
 
     void requestStyleSheet();
 
     void traceAfterDispatch(Visitor*);
@@ skipping 27 matching lines @@
     RefPtrWillBeMember<StyleSheetContents> m_styleSheet;
     ResourcePtr<CSSStyleSheetResource> m_resource;
     bool m_loading;
 };
 
 DEFINE_STYLE_RULE_TYPE_CASTS(Import);
 
 } // namespace WebCore
 
 #endif