Add a spec link for the Iterator interface

Source/core/dom/Iterator.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// http://people.mozilla.org/~jorendorff/es6-draft.html#sec-iterator-interface
+
 [
     Iterable,
     GarbageCollected,
     NoInterfaceObject,
 ] interface Iterator {
     [CallWith=ScriptState, RaisesException] any next(optional any value);
 };