ChromeOS OOBE: Add ARIA-labels for OOBE screens.

chrome/browser/resources/chromeos/login/oobe_buttons.html

 <!-- 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. -->
 
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-iconset-svg/iron-iconset-svg.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-fab/paper-fab.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 
@@ skipping 19 matching lines @@
   Note, that content must have separate DOM node (i.e. a separate div).
 
   Example:
     <oobe-icon-button inverse>
       <div>OK</div>
     </oobe-icon-button>
 
   Attributes:
     'disabled' - button is disabled when the attribute is set.
     'inverse' - makes text white and background blue
+    'aria-label' - accessibility label.
 -->
 <dom-module id="oobe-text-button">
   <link rel="stylesheet" href="oobe_text_button.css">
   <template>
     <paper-button id="textButton" on-tap="onClick_" disabled="[[disabled]]"
-        inverse$="[[inverse]]">
+        inverse$="[[inverse]]" aria-label$="[[ariaLabel]]">
       <div id="container"
           class="flex layout horizontal center center-justified self-stretch">
         <content></content>
       </div>
     </paper-button>
   </template>
 </dom-module>
 
 <!--
   Material design square "<- Back" and "Next ->" buttons.
   Text is blue, background is white.
 
   Example:
     <oobe-back-button on-tap="handleOnTap_">
     </oobe-back-button>
 
   Attributes:
     'disabled' - button is disabled when the attribute is set.
+    'aria-label' - accessibility label.
 -->
 <dom-module id="oobe-back-button">
   <link rel="stylesheet" href="oobe_nav_button.css">
   <template>
-    <paper-button id="button" on-tap="onClick_" disabled="[[disabled]]">
+    <paper-button id="button" on-tap="onClick_" disabled="[[disabled]]"
+        aria-label$="[[ariaLabel]]">
       <div class="flex horizontal layout start center">
         <iron-icon icon="oobe-buttons:arrow-back"></iron-icon>
         <div id="text">$i18n{back}</div>
       </div>
     </paper-button>
   </template>
 </dom-module>
 
 <dom-module id="oobe-next-button">
   <link rel="stylesheet" href="oobe_nav_button.css">
@@ skipping 15 matching lines @@
       <div>i18n version of oobeCloseButtonText</div>
     </oobe-welcome-secondary-button>
 
   Attributes:
     'icon' - a name of icon from material design set to show on button.
     'aria-label' - accessibility label.
 -->
 <dom-module id="oobe-welcome-secondary-button">
   <link rel="stylesheet" href="oobe_welcome_secondary_button.css">
   <template>
-    <paper-button id="button" aria-label="[[ariaLabel]]">
+    <paper-button id="button" aria-label$="[[ariaLabel]]">
       <div id="container" class="flex layout vertical center self-stretch">
         <div id="subcontainer"
             class="flex layout horizontal center self-stretch">
           <iron-icon icon="[[icon]]" class="oobe-icon"></iron-icon>
           <content></content>
         </div>
       </div>
     </paper-button>
   </template>
 </dom-module>