Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/dom/ProcessingInstruction.cpp

 /*
  * Copyright (C) 2000 Peter Kelly (pmk@post.com)
  * Copyright (C) 2006, 2008, 2009 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 226 matching lines @@
     if (!insertionPoint->inDocument())
         return;
 
     document().styleEngine()->removeStyleSheetCandidateNode(this);
 
     RefPtr<StyleSheet> removedSheet = m_sheet;
 
     if (m_sheet) {
         ASSERT(m_sheet->ownerNode() == this);
         m_sheet->clearOwnerNode();
-        m_sheet = 0;
+        m_sheet = nullptr;
     }
 
     // If we're in document teardown, then we don't need to do any notification of our sheet's removal.
     if (document().isActive())
         document().removedStyleSheet(removedSheet.get());
 }
 
 } // namespace