Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/graphics/ImageSource.cpp

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp.toker@collabora.co.uk>
  * Copyright (C) 2008, 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.
@@ skipping 95 matching lines @@
 }
 
 size_t ImageSource::frameCount() const
 {
     return m_decoder ? m_decoder->frameCount() : 0;
 }
 
 PassRefPtr<NativeImageSkia> ImageSource::createFrameAtIndex(size_t index)
 {
     if (!m_decoder)
-        return 0;
+        return nullptr;
 
     ImageFrame* buffer = m_decoder->frameBufferAtIndex(index);
     if (!buffer || buffer->status() == ImageFrame::FrameEmpty)
-        return 0;
+        return nullptr;
 
     // Zero-height images can cause problems for some ports.  If we have an
     // empty image dimension, just bail.
     if (size().isEmpty())
-        return 0;
+        return nullptr;
 
     // Return the buffer contents as a native image.  For some ports, the data
     // is already in a native container, and this just increments its refcount.
     return buffer->asNewNativeImage();
 }
 
 float ImageSource::frameDurationAtIndex(size_t index) const
 {
     if (!m_decoder)
         return 0;
@@ skipping 24 matching lines @@
 }
 
 unsigned ImageSource::frameBytesAtIndex(size_t index) const
 {
     if (!m_decoder)
         return 0;
     return m_decoder->frameBytesAtIndex(index);
 }
 
 }