Drop [TreatNullAs=NullString] for Document.designMode

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * 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 102 matching lines @@
     // FIXME: There are two open() methods in the spec.
     [Custom, CustomElementCallbacks, RaisesException] void open();
     [RaisesException] void close();
     [CallWith=ActiveWindow, CustomElementCallbacks, RaisesException] void write(DOMString... text);
     [CallWith=ActiveWindow, CustomElementCallbacks, RaisesException] void writeln(DOMString... text);
 
     // user interaction
     [ImplementedAs=domWindow] readonly attribute Window? defaultView;
     readonly attribute Element? activeElement;
     boolean hasFocus();
-    // FIXME: designMode should not have [TreatNullAs=NullString].
-    [TreatNullAs=NullString, CustomElementCallbacks, MeasureAs=DocumentDesignMode] attribute DOMString designMode;
+    [CustomElementCallbacks, MeasureAs=DocumentDesignMode] attribute DOMString designMode;
     [CustomElementCallbacks, RaisesException] boolean execCommand(DOMString commandId, optional boolean showUI = false, optional DOMString value = "");
     [RaisesException] boolean queryCommandEnabled(DOMString commandId);
     [RaisesException] boolean queryCommandIndeterm(DOMString commandId);
     [RaisesException] boolean queryCommandState(DOMString commandId);
     [RaisesException] boolean queryCommandSupported(DOMString commandId);
     [RaisesException] DOMString queryCommandValue(DOMString commandId);
 
     [LenientThis] attribute EventHandler onreadystatechange;
 
     // HTML obsolete features
@@ skipping 88 matching lines @@
     attribute EventHandler onsearch;
     [RuntimeEnabled=ExperimentalContentSecurityPolicyFeatures] attribute EventHandler onsecuritypolicyviolation;
     attribute EventHandler onselectionchange;
     attribute EventHandler onselectstart;
     attribute EventHandler onwheel;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;
 Document implements NonElementParentNode;