Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/core/html/HTMLInputElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * 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.
  *
@@ skipping 54 matching lines @@
     [RaisesException=Setter, CustomElementCallbacks] attribute unrestricted double valueAsNumber;
 
     [RaisesException, CustomElementCallbacks] void stepUp(optional long n);
     [RaisesException, CustomElementCallbacks] void stepDown(optional long n);
 
     [CustomElementCallbacks] attribute unsigned long width;
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
-    void setCustomValidity([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString error);
+    void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
 
     readonly attribute NodeList labels;
 
     void select();
     [RaisesException, ImplementedAs=selectionStartForBinding] attribute long selectionStart;
     [RaisesException, ImplementedAs=selectionEndForBinding] attribute long selectionEnd;
     [RaisesException, ImplementedAs=selectionDirectionForBinding] attribute DOMString selectionDirection;
 
     [RaisesException] void setRangeText(DOMString replacement);
     [RaisesException] void setRangeText(DOMString replacement,
                         unsigned long start,
                         unsigned long end,
                         optional DOMString selectionMode = null);
 
     [RaisesException, ImplementedAs=setSelectionRangeForBinding]
     void setSelectionRange([Default=Undefined] optional long start,
                            [Default=Undefined] optional long end,
                            optional DOMString direction);
 
     // Non-standard attributes
     [Reflect, MeasureAs=PrefixedDirectoryAttribute] attribute boolean webkitdirectory;
     [Reflect, MeasureAs=IncrementalAttribute] attribute boolean incremental;
 
     // See http://www.w3.org/TR/html-media-capture/
     [RuntimeEnabled=MediaCapture, Reflect] attribute boolean capture;
 };