Make the setCustomValidity() error argument not 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 55 matching lines @@
     [CustomElementCallbacks] attribute unsigned long width;
 
     [RaisesException, CustomElementCallbacks] void stepUp(optional long n = 1);
     [RaisesException, CustomElementCallbacks] void stepDown(optional long n = 1);
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
-    // FIXME: The error argument should not be nullable.
-    void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
+    void setCustomValidity(DOMString error);
 
     readonly attribute NodeList labels;
 
     void select();
     // FIXME: selectionStart and selectionEnd should be unsigned long.
     [RaisesException, ImplementedAs=selectionStartForBinding] attribute long selectionStart;
     [RaisesException, ImplementedAs=selectionEndForBinding] attribute long selectionEnd;
     [RaisesException, ImplementedAs=selectionDirectionForBinding] attribute DOMString selectionDirection;
     [RaisesException] void setRangeText(DOMString replacement);
     // FIXME: The selectionMode argument should be a SelectionMode enum.
@@ skipping 17 matching lines @@
     [Measure] attribute DOMString autocapitalize;
 
     // HTML Media Capture
     // http://www.w3.org/TR/html-media-capture/
     [RuntimeEnabled=MediaCapture, Reflect] attribute boolean capture;
 
     // Non-standard APIs
     [Reflect, MeasureAs=PrefixedDirectoryAttribute] attribute boolean webkitdirectory;
     [Reflect, MeasureAs=IncrementalAttribute] attribute boolean incremental;
 };