Remove tabStop feature

Source/core/dom/Element.idl

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@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,
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     // FIXME: dataset should be on HTMLElement.
     [SameObject, PerWorldBindings] readonly attribute DOMStringMap dataset;
 
     // Non-standard APIs
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=19962
     [RaisesException, CustomElementCallbacks, MeasureAs=InsertAdjacentElement] Element insertAdjacentElement(DOMString where, Element element);
     [RaisesException, MeasureAs=InsertAdjacentText] void insertAdjacentText(DOMString where, DOMString text);
     [MeasureAs=ElementScrollIntoViewIfNeeded] void scrollIntoViewIfNeeded(optional boolean centerIfNeeded);
     [RaisesException, ImplementedAs=matches, MeasureAs=ElementPrefixedMatchesSelector] boolean webkitMatchesSelector(DOMString selectors);
 
-    // Experimental isTapStop API
-    [RuntimeEnabled=TabStopAttribute, MeasureAs=TabStopProperty] attribute boolean tabStop;
-
     // Experimental accessibility API
     [RuntimeEnabled=ComputedAccessibilityInfo] readonly attribute DOMString? computedRole;
     [RuntimeEnabled=ComputedAccessibilityInfo] readonly attribute DOMString? computedName;
 
     // Event handler attributes
     attribute EventHandler onbeforecopy;
     attribute EventHandler onbeforecut;
     attribute EventHandler onbeforepaste;
     attribute EventHandler oncopy;
     attribute EventHandler oncut;
     attribute EventHandler onpaste;
     attribute EventHandler onsearch;
     attribute EventHandler onselectstart;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchcancel;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchend;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchmove;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchstart;
     attribute EventHandler onwheel;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;