Implement 'tabstop' as an HTML attribute

Source/core/dom/ElementRareData.h

Index: Source/core/dom/ElementRareData.h
diff --git a/Source/core/dom/ElementRareData.h b/Source/core/dom/ElementRareData.h
index d314723c3a50b1e9ead9e83cf6f81dc37eb6b9ae..a568285e7223e45f64c4f2fe45d321e1ae45f7c0 100644
--- a/Source/core/dom/ElementRareData.h
+++ b/Source/core/dom/ElementRareData.h
@@ -57,15 +57,12 @@ public:
     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);
     }