Use C++11 features in core/html

Source/core/html/HTMLLabelElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * 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
@@ skipping 50 matching lines @@
 {
     const AtomicString& controlId = getAttribute(forAttr);
     if (controlId.isNull()) {
         // Search the children and descendants of the label element for a form element.
         // per http://dev.w3.org/html5/spec/Overview.html#the-label-element
         // the form element must be "labelable form-associated element".
         for (LabelableElement& element : Traversal<LabelableElement>::descendantsOf(*this)) {
             if (element.supportLabels())
                 return &element;
         }
-        return 0;
+        return nullptr;
     }
 
     if (Element* element = treeScope().getElementById(controlId)) {
         if (isLabelableElement(*element) && toLabelableElement(*element).supportLabels())
             return toLabelableElement(element);
     }
 
-    return 0;
+    return nullptr;
 }
 
 HTMLFormElement* HTMLLabelElement::formOwner() const
 {
     return FormAssociatedElement::form();
 }
 
 void HTMLLabelElement::setActive(bool down)
 {
     if (down == active())
@@ skipping 186 matching lines @@
 
 void HTMLLabelElement::parseAttribute(const QualifiedName& attributeName, const AtomicString& attributeValue)
 {
     if (attributeName == formAttr)
         formAttributeChanged();
     else
         HTMLElement::parseAttribute(attributeName, attributeValue);
 }
 
 } // namespace