Use C++11 features in core/html

Source/core/html/HTMLElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004-2007, 2009, 2014 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 56 matching lines @@
 
     const AtomicString& dir();
     void setDir(const AtomicString&);
 
     void click();
 
     virtual void accessKeyAction(bool sendMouseEvents) override;
 
     bool ieForbidsInsertHTML() const;
 
-    virtual HTMLFormElement* formOwner() const { return 0; }
+    virtual HTMLFormElement* formOwner() const { return nullptr; }
 
     HTMLFormElement* findFormAncestor() const;
 
     bool hasDirectionAuto() const;
     TextDirection directionalityIfhasDirAutoAttribute(bool& isAuto) const;
 
     virtual bool isHTMLUnknownElement() const { return false; }
     virtual bool isPluginElement() const { return false; }
 
     virtual bool isLabelable() const { return false; }
@@ skipping 85 matching lines @@
     template<typename T> inline bool is##thisType(const PassRefPtr<T>& node) { return is##thisType(node.get()); } \
     template<typename T> inline bool is##thisType(const RefPtr<T>& node) { return is##thisType(node.get()); } \
     template <> inline bool isElementOfType<const thisType>(const HTMLElement& element) { return is##thisType(element); } \
     DEFINE_ELEMENT_TYPE_CASTS_WITH_FUNCTION(thisType)
 
 } // namespace blink
 
 #include "core/HTMLElementTypeHelpers.h"
 
 #endif // HTMLElement_h