Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/resolver/ElementStyleResources.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google 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 36 matching lines @@
             return generatedOrPendingFromValue(property, toCSSGradientValue(value)->gradientWithStylesResolved(textLinkColors, currentColor).get());
         return generatedOrPendingFromValue(property, toCSSImageGeneratorValue(value));
     }
 
     if (value->isImageSetValue())
         return setOrPendingFromValue(property, toCSSImageSetValue(value));
 
     if (value->isCursorImageValue())
         return cursorOrPendingFromValue(property, toCSSCursorImageValue(value));
 
-    return 0;
+    return nullptr;
 }
 
 PassRefPtr<StyleImage> ElementStyleResources::generatedOrPendingFromValue(CSSPropertyID property, CSSImageGeneratorValue* value)
 {
     if (value->isPending()) {
         m_pendingImageProperties.set(property, value);
         return StylePendingImage::create(value);
     }
     return StyleGeneratedImage::create(value);
 }
@@ skipping 21 matching lines @@
         m_pendingImageProperties.set(property, value);
     return image.release();
 }
 
 void ElementStyleResources::addPendingSVGDocument(FilterOperation* filterOperation, CSSSVGDocumentValue* cssSVGDocumentValue)
 {
     m_pendingSVGDocuments.set(filterOperation, cssSVGDocumentValue);
 }
 
 }