Use nullptr instead of 0 in xml and xmlhttprequest.

Source/core/xml/XPathNodeSet.cpp

 /*
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  *
  * 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 245 matching lines @@
     while (from < to) {
         m_nodes[from].swap(m_nodes[to]);
         ++from;
         --to;
     }
 }
 
 Node* NodeSet::firstNode() const
 {
     if (isEmpty())
-        return 0;
+        return nullptr;
 
     // FIXME: fully sorting the node-set just to find its first node is
     // wasteful.
     sort();
     return m_nodes.at(0).get();
 }
 
 Node* NodeSet::anyNode() const
 {
     if (isEmpty())
-        return 0;
+        return nullptr;
 
     return m_nodes.at(0).get();
 }
 
 }
 }