Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/css/CSSStyleDeclaration.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 11 matching lines @@
 [
     DependentLifetime,
     WillBeGarbageCollected
 ] interface CSSStyleDeclaration {
     [RaisesException=Setter] attribute DOMString?       cssText;
 
     DOMString?          getPropertyValue([Default=Undefined] optional DOMString propertyName);
     [RaisesException] DOMString?          removeProperty([Default=Undefined] optional DOMString propertyName);
     DOMString?          getPropertyPriority([Default=Undefined] optional DOMString propertyName);
     [RaisesException] void setProperty([Default=Undefined] optional DOMString propertyName,
-                                       [TreatNullAs=NullString,Default=Undefined] optional DOMString value,
-                                       [TreatNullAs=NullString] optional DOMString priority = null);
+                                       [Default=Undefined] optional DOMString? value,
+                                       optional DOMString? priority = null);
 
     readonly attribute unsigned long    length;
     getter DOMString          item([Default=Undefined] optional unsigned long index);
     [Custom=PropertyGetter|PropertyEnumerator|PropertyQuery] getter (DOMString or float) (DOMString name);
-    [Custom] setter void (DOMString propertyName, [TreatNullAs=NullString] DOMString propertyValue);
+    [Custom] setter void (DOMString propertyName, DOMString? propertyValue);
     readonly attribute CSSRule          parentRule;
 
     // Deprecated as of 2003: http://lists.w3.org/Archives/Public/www-style/2003Oct/0347.html
     // FIXME: This should be DeprecateAs=, but currently too many LayoutTests use
     // this function and would need CONSOLE: message rebaselines!
     [MeasureAs=CSSStyleDeclarationGetPropertyCSSValue] CSSValue           getPropertyCSSValue([Default=Undefined] optional DOMString propertyName);
 };