Gen index getter by unified way instead of using several helper function in some special cases.

Source/WebCore/html/HTMLFormElement.idl

 /*
  * Copyright (C) 2006, 2010 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 [
-    IndexedGetter,
     CustomIndexedGetter,
     CustomNamedGetter
 ] interface HTMLFormElement : HTMLElement {
     [Reflect=accept_charset] attribute DOMString acceptCharset;
     [Reflect, URL] attribute DOMString action;
     [Reflect] attribute DOMString autocomplete;
     [TreatNullAs=NullString] attribute DOMString enctype;
     [TreatNullAs=NullString] attribute DOMString encoding;
     [TreatNullAs=NullString] attribute DOMString method;
     [Reflect] attribute DOMString name;
     [Reflect] attribute boolean noValidate;
     [Reflect] attribute DOMString target;
 
     readonly attribute HTMLCollection elements;
     readonly attribute long length;
 
     [ImplementedAs=submitFromJavaScript] void submit();
     void reset();
     boolean checkValidity();
 
     [EnabledAtRuntime=requestAutocomplete] void requestAutocomplete();
     [EnabledAtRuntime=requestAutocomplete,NotEnumerable] attribute EventListener onautocomplete;
     [EnabledAtRuntime=requestAutocomplete,NotEnumerable] attribute EventListener onautocompleteerror;
 };