Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/web/tests/DragImageTest.cpp

 /*
  * Copyright (C) 2010 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ 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 bool currentFrameKnownToBeOpaque() OVERRIDE
@@ skipping 64 matching lines @@
 
     OwnPtr<DragImage> testImage =
         DragImage::create(url, testLabel, fontDescription, deviceScaleFactor);
     OwnPtr<DragImage> expectedImage =
         DragImage::create(url, expectedLabel, fontDescription, deviceScaleFactor);
 
     EXPECT_EQ(testImage->size().width(), expectedImage->size().width());
 }
 
 } // anonymous namespace