Implement 'tabstop' as an HTML attribute

Source/core/dom/ElementRareData.h

 /*
  * Copyright (C) 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 David Smith <catfish.man@gmail.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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 39 matching lines @@
     ~ElementRareData();
 
     void setPseudoElement(PseudoId, PassRefPtrWillBeRawPtr<PseudoElement>);
     PseudoElement* pseudoElement(PseudoId) const;
 
     short tabIndex() const { return m_tabindex; }
 
     void setTabIndexExplicitly(short index)
     {
         m_tabindex = index;
-        // tabStop is overridden by setting tabindex.
-        m_tabStop = (m_tabindex >= 0);
         setElementFlag(TabIndexWasSetExplicitly, true);
     }
 
     void clearTabIndexExplicitly()
     {
         m_tabindex = 0;
-        m_tabStop = true;
         clearElementFlag(TabIndexWasSetExplicitly);
     }
 
     bool tabStop() const { return m_tabStop; }
 
     void setTabStop(bool flag) { m_tabStop = flag; }
 
     CSSStyleDeclaration& ensureInlineCSSStyleDeclaration(Element* ownerElement);
 
     void clearShadow() { m_shadow = nullptr; }
@@ skipping 161 matching lines @@
     case FIRST_LETTER:
         return m_generatedFirstLetter.get();
     default:
         return 0;
     }
 }
 
 } // namespace
 
 #endif // ElementRareData_h