Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/dom/Position.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2009 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 270 matching lines @@
     Node* n = anchorNode();
     while (n && !n->isElementNode())
         n = n->parentNode();
     return toElement(n);
 }
 
 PassRefPtr<CSSComputedStyleDeclaration> Position::computedStyle() const
 {
     Element* elem = element();
     if (!elem)
-        return 0;
+        return nullptr;
     return CSSComputedStyleDeclaration::create(elem);
 }
 
 Position Position::previous(PositionMoveType moveType) const
 {
     Node* node = deprecatedNode();
     if (!node)
         return *this;
 
     int offset = deprecatedEditingOffset();
@@ skipping 1080 matching lines @@
     pos.showTreeForThis();
 }
 
 void showTree(const WebCore::Position* pos)
 {
     if (pos)
         pos->showTreeForThis();
 }
 
 #endif