Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/fetch/DocumentResource.cpp

 /*
     Copyright (C) 2010 Rob Buis <rwlbuis@gmail.com>
     Copyright (C) 2011 Cosmin Truta <ctruta@gmail.com>
     Copyright (C) 2012 University of Szeged
     Copyright (C) 2012 Renata Hodovan <reni@webkit.org>
 
     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 55 matching lines @@
 }
 
 PassRefPtr<Document> DocumentResource::createDocument(const KURL& url)
 {
     switch (type()) {
     case SVGDocument:
         return SVGDocument::create(DocumentInit(url));
     default:
         // FIXME: We'll add more types to support HTMLImports.
         ASSERT_NOT_REACHED();
-        return 0;
+        return nullptr;
     }
 }
 
 }