Revert "Reverting custom pseudo element".

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,
@@ skipping 93 matching lines @@
     [PerWorldBindings] readonly attribute DOMStringMap dataset;
 
     // NodeSelector - Selector API
     [RaisesException] Element querySelector(DOMString selectors);
     [RaisesException] NodeList querySelectorAll(DOMString selectors);
 
     // WebKit extension
     [RaisesException, ImplementedAs=matches, MeasureAs=ElementPrefixedMatchesSelector] boolean webkitMatchesSelector(DOMString selectors);
 
     // Shadow DOM API
+    [RuntimeEnabled=ShadowDOM, Reflect, TreatNullAs=NullString, PerWorldBindings] attribute DOMString pseudo;
     [RuntimeEnabled=ShadowDOM, RaisesException] ShadowRoot createShadowRoot();
     [RuntimeEnabled=ShadowDOM, PerWorldBindings] readonly attribute ShadowRoot shadowRoot;
     [RuntimeEnabled=ShadowDOM, PerWorldBindings] NodeList getDestinationInsertionPoints();
 
     // To-be-deprecated prefixed Shadow DOM API
     [ImplementedAs=createShadowRoot, RaisesException, MeasureAs=ShadowDOMPrefixedCreateShadowRoot] ShadowRoot webkitCreateShadowRoot();
     [ImplementedAs=shadowRoot, PerWorldBindings, MeasureAs=ShadowDOMPrefixedShadowRoot] readonly attribute ShadowRoot webkitShadowRoot;
 
     // CSSOM View Module API
     ClientRectList getClientRects();
@@ skipping 21 matching lines @@
     [RuntimeEnabled=Touch] attribute EventHandler ontouchend;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchmove;
     [RuntimeEnabled=Touch] attribute EventHandler ontouchstart;
     attribute EventHandler onwebkitfullscreenchange;
     attribute EventHandler onwebkitfullscreenerror;
     [PerWorldBindings, ActivityLogging=SetterForIsolatedWorlds] attribute EventHandler onwheel;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;