Use nullptr instead of 0 in xml and xmlhttprequest.

Source/core/xml/XSLStyleSheetLibxslt.cpp

 /*
  * This file is part of the XSL implementation.
  *
  * Copyright (C) 2004, 2005, 2006, 2008, 2012 Apple Inc. 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 251 matching lines @@
 
 Document* XSLStyleSheet::ownerDocument()
 {
     for (XSLStyleSheet* styleSheet = this; styleSheet; styleSheet = styleSheet->parentStyleSheet()) {
         if (styleSheet->m_ownerDocument)
             return styleSheet->m_ownerDocument.get();
         Node* node = styleSheet->ownerNode();
         if (node)
             return &node->document();
     }
-    return 0;
+    return nullptr;
 }
 
 xmlDocPtr XSLStyleSheet::locateStylesheetSubResource(xmlDocPtr parentDoc, const xmlChar* uri)
 {
     bool matchedParent = (parentDoc == document());
     for (unsigned i = 0; i < m_children.size(); ++i) {
         XSLImportRule* import = m_children.at(i).get();
         XSLStyleSheet* child = import->styleSheet();
         if (!child)
             continue;
@@ skipping 36 matching lines @@
 DEFINE_TRACE(XSLStyleSheet)
 {
     visitor->trace(m_ownerNode);
     visitor->trace(m_children);
     visitor->trace(m_parentStyleSheet);
     visitor->trace(m_ownerDocument);
     StyleSheet::trace(visitor);
 }
 
 } // namespace blink