Use DOMSettableTokenList for {HTMLAnchorElement, HTMLAreaElement}.ping.

third_party/WebKit/Source/core/html/HTMLAreaElement.idl

 /*
  * Copyright (C) 2006, 2009, 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.
  */
 
 // https://html.spec.whatwg.org/#the-area-element
 
 interface HTMLAreaElement : HTMLElement {
     [Reflect] attribute DOMString alt;
     [Reflect] attribute DOMString coords;
     [Reflect] attribute DOMString shape;
     [Reflect] attribute DOMString target;
-    // FIXME: ping should be a DOMSettableTokenList.
-    [Reflect] attribute DOMString ping;
+    [PutForwards=value] attribute DOMSettableTokenList ping;
     // FIXME: download, rel, relList are missing
 
     // obsolete members
     // https://html.spec.whatwg.org/#HTMLAreaElement-partial
     [Reflect] attribute boolean noHref;
 };
 
 HTMLAreaElement implements URLUtils;