Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/web/tests/ImageLayerChromiumTest.cpp

 /*
  * Copyright (C) 2011 Google 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 59 matching lines @@
     }
 
     virtual IntSize size() const OVERRIDE
     {
         return m_size;
     }
 
     virtual PassRefPtr<NativeImageSkia> nativeImageForCurrentFrame() OVERRIDE
     {
         if (m_size.isZero())
-            return 0;
+            return nullptr;
 
         return m_nativeImage;
     }
 
     // Stub implementations of pure virtual Image functions.
     virtual void destroyDecodedData(bool) OVERRIDE
     {
     }
 
     virtual void draw(GraphicsContext*, const FloatRect&, const FloatRect&, CompositeOperator, blink::WebBlendMode) OVERRIDE
@@ skipping 29 matching lines @@
     ASSERT_FALSE(graphicsLayer->contentsLayer());
 
     graphicsLayer->setContentsToImage(opaqueImage.get());
     ASSERT_TRUE(graphicsLayer->contentsLayer()->opaque());
 
     graphicsLayer->setContentsToImage(nonOpaqueImage.get());
     ASSERT_FALSE(graphicsLayer->contentsLayer()->opaque());
 }
 
 } // namespace