Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

Source/core/css/CSSValue.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 12 matching lines @@
     DependentLifetime,
     WillBeGarbageCollected,
 ] interface CSSValue {
 
     // UnitTypes
     const unsigned short CSS_INHERIT         = 0;
     const unsigned short CSS_PRIMITIVE_VALUE = 1;
     const unsigned short CSS_VALUE_LIST      = 2;
     const unsigned short CSS_CUSTOM          = 3;
 
-             [TreatReturnedNullStringAs=Null, TreatNullAs=NullString, RaisesException=Setter] attribute DOMString cssText;
+    [RaisesException=Setter] attribute DOMString? cssText;
 
     readonly attribute unsigned short cssValueType;
 
 };