ChromeOS OOBE: fix focus flow in MD-OOBE.

chrome/browser/resources/chromeos/login/oobe_welcome_dialog.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-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 
 <!--
   OOBE Welcome screen
 -->
 <dom-module name="oobe-welcome-dialog">
   <template>
     <link rel="stylesheet" href="oobe_dialog_host.css">
     <link rel="stylesheet" href="oobe_welcome_dialog.css">
     <div class="layout horizontal flex">
       <div class="vertical-margin"></div>
       <div class="welcome-content flex layout vertical center">
         <h1 id="title">$i18n{networkScreenGreeting}</h1>
         <!-- TODO(alemate): Display images depending on screen ppi. -->
         <img id="welcomeIllustration" src="images/welcome_illustration_2x.png">
         <div class="flex"></div>
         <div id="buttons" class="layout horizontal center">
-          <oobe-welcome-secondary-button icon="icons:language"
-              on-tap="onLanguageClicked_"
+          <oobe-welcome-secondary-button id="languageSelectionButton"
+              icon="icons:language" on-tap="onLanguageClicked_"
               aria-label="[[formatMessage_('languageButtonLabel',
                   currentLanguage)]]">
             <div>[[currentLanguage]]</div>
           </oobe-welcome-secondary-button>
-          <oobe-welcome-secondary-button icon="icons:accessibility"
-              on-tap="onAccessibilityClicked_">
+          <oobe-welcome-secondary-button id="accessibilitySettingsButton"
+              icon="icons:accessibility" on-tap="onAccessibilityClicked_">
             <div>$i18n{accessibilityLink}</div>
           </oobe-welcome-secondary-button>
-          <oobe-welcome-secondary-button icon="oobe-welcome-64:timezone"
-              on-tap="onTimezoneClicked_" hidden="[[!timezoneButtonVisible]]">
+          <oobe-welcome-secondary-button id="timezoneSettingsButton"
+              icon="oobe-welcome-64:timezone" on-tap="onTimezoneClicked_"
+              hidden="[[!timezoneButtonVisible]]">
             <div>$i18n{timezoneButtonText}</div>
           </oobe-welcome-secondary-button>
           <div class="flex"></div>
-          <oobe-text-button inverse on-tap="onNextClicked_">
+          <oobe-text-button id="welcomeNextButton"
+              inverse on-tap="onNextClicked_">
             <div>$i18n{welcomeNextButtonText}</div>
           </oobe-text-button>
         </div>
         <div id="bottom-margin"></div>
       </div>
       <div class="vertical-margin"></div>
     </div>
   </template>
 </dom-module>