Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/CSSCrossfadeValue.cpp

 /*
  * Copyright (C) 2011 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 149 matching lines @@
         if (m_cachedToImage)
             m_cachedToImage->addClient(&m_crossfadeSubimageObserver);
     }
 
     m_crossfadeSubimageObserver.setReady(true);
 }
 
 PassRefPtr<Image> CSSCrossfadeValue::image(RenderObject* renderer, const IntSize& size)
 {
     if (size.isEmpty())
-        return 0;
+        return nullptr;
 
     ResourceFetcher* fetcher = renderer->document().fetcher();
     ImageResource* cachedFromImage = cachedImageForCSSValue(m_fromValue.get(), fetcher);
     ImageResource* cachedToImage = cachedImageForCSSValue(m_toValue.get(), fetcher);
 
     if (!cachedFromImage || !cachedToImage)
         return Image::nullImage();
 
     Image* fromImage = cachedFromImage->imageForRenderer(renderer);
     Image* toImage = cachedToImage->imageForRenderer(renderer);
@@ skipping 39 matching lines @@
 
 void CSSCrossfadeValue::traceAfterDispatch(Visitor* visitor)
 {
     visitor->trace(m_fromValue);
     visitor->trace(m_toValue);
     visitor->trace(m_percentageValue);
     CSSImageGeneratorValue::traceAfterDispatch(visitor);
 }
 
 } // namespace WebCore