Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/StylePropertySerializer.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Research In Motion Limited. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 552 matching lines @@
             RefPtrWillBeRawPtr<CSSValue> value;
             if (values[j]) {
                 if (values[j]->isBaseValueList())
                     value = toCSSValueList(values[j].get())->item(i);
                 else {
                     value = values[j];
 
                     // Color only belongs in the last layer.
                     if (shorthand.properties()[j] == CSSPropertyBackgroundColor) {
                         if (i != numLayers - 1)
-                            value = 0;
-                    } else if (i) // Other singletons only belong in the first layer.
-                        value = 0;
+                            value = nullptr;
+                    } else if (i) {
+                        // Other singletons only belong in the first layer.
+                        value = nullptr;
+                    }
                 }
             }
 
             // We need to report background-repeat as it was written in the CSS. If the property is implicit,
             // then it was written with only one value. Here we figure out which value that was so we can
             // report back correctly.
             if ((shorthand.properties()[j] == CSSPropertyBackgroundRepeatX && m_propertySet.isPropertyImplicit(shorthand.properties()[j]))
                 || (shorthand.properties()[j] == CSSPropertyWebkitMaskRepeatX && m_propertySet.isPropertyImplicit(shorthand.properties()[j]))) {
 
                 // BUG 49055: make sure the value was not reset in the layer check just above.
@@ skipping 167 matching lines @@
         if (!result.isEmpty())
             result.append(' ');
         result.append(value);
     }
     if (isInitialOrInherit(commonValue))
         return commonValue;
     return result.isEmpty() ? String() : result.toString();
 }
 
 }