Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/graphics/GraphicsContext.cpp

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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
@@ skipping 1890 matching lines @@
         return ImageBuffer::createColorSpaceFilter(ColorSpaceLinearRGB, ColorSpaceDeviceRGB);
     case ColorFilterSRGBToLinearRGB:
         return ImageBuffer::createColorSpaceFilter(ColorSpaceDeviceRGB, ColorSpaceLinearRGB);
     case ColorFilterNone:
         break;
     default:
         ASSERT_NOT_REACHED();
         break;
     }
 
-    return 0;
+    return nullptr;
 }
 
 #if OS(MACOSX)
 CGColorSpaceRef PLATFORM_EXPORT deviceRGBColorSpaceRef()
 {
     static CGColorSpaceRef deviceSpace = CGColorSpaceCreateDeviceRGB();
     return deviceSpace;
 }
 #else
 void GraphicsContext::draw2xMarker(SkBitmap* bitmap, int index)
@@ skipping 110 matching lines @@
 
 void GraphicsContext::didDrawTextInRect(const SkRect& textRect)
 {
     if (m_trackTextRegion) {
         TRACE_EVENT0("skia", "PlatformContextSkia::trackTextRegion");
         m_textRegion.join(textRect);
     }
 }
 
 }