Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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 377 matching lines @@
 
     if (m_context)
         m_context->paintRenderingResultsToCanvas();
 
     return buffer()->toDataURL(encodingMimeType, quality);
 }
 
 PassRefPtr<ImageData> HTMLCanvasElement::getImageData()
 {
     if (!m_context || !m_context->is3d())
-       return 0;
+        return nullptr;
     return toWebGLRenderingContext(m_context.get())->paintRenderingResultsToImageData();
 }
 
 SecurityOrigin* HTMLCanvasElement::securityOrigin() const
 {
     return document().securityOrigin();
 }
 
 bool HTMLCanvasElement::shouldAccelerate(const IntSize& size) const
 {
@@ skipping 184 matching lines @@
     }
 }
 
 void HTMLCanvasElement::didMoveToNewDocument(Document& oldDocument)
 {
     setObservedDocument(document());
     HTMLElement::didMoveToNewDocument(oldDocument);
 }
 
 }