Consistently use AtomicString for namespaceURI / prefix

Source/core/xml/NativeXPathNSResolver.cpp

 /*
  * Copyright (C) 2007 Alexey Proskuryakov (ap@nypop.com)
  *
  * 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 23 matching lines @@
 
 NativeXPathNSResolver::NativeXPathNSResolver(PassRefPtr<Node> node)
     : m_node(node)
 {
 }
 
 NativeXPathNSResolver::~NativeXPathNSResolver()
 {
 }
 
-String NativeXPathNSResolver::lookupNamespaceURI(const String& prefix)
+AtomicString NativeXPathNSResolver::lookupNamespaceURI(const String& prefix)
 {
     // This is not done by Node::lookupNamespaceURI as per the DOM3 Core spec,
     // but the XPath spec says that we should do it for XPathNSResolver.
     if (prefix == "xml")
         return XMLNames::xmlNamespaceURI;
 
-    return m_node ? m_node->lookupNamespaceURI(prefix) : String();
+    return m_node ? m_node->lookupNamespaceURI(prefix) : nullAtom;
 }
 
 } // namespace WebCore