ChromeOS OOBE: Add ARIA-labels for OOBE screens.

chrome/browser/resources/chromeos/login/oobe_a11y_option.js

 // Copyright 2016 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.
 
 Polymer({
   is: 'oobe-a11y-option',
 
   properties: {
     /**
      * If paper-toggle-button is checked.
      */
     checked: {
       type: Boolean,
     },
 
     /**
      * Chrome message handling this option.
      */
     chromeMessage: {
       type: String,
     },
+
+    /**
+     * ARIA-label for the button.
+     */
+    ariaLabel: String,
   },
 });