ChromeOS OOBE: fix focus flow in MD-OOBE.

chrome/browser/resources/chromeos/login/oobe_dialog.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-dialog',
 
   properties: {
     /**
      * Controls visibility of the bottom-buttons element.
      */
     hasButtons: {
       type: Boolean,
       value: false,
     },
 
     /**
      * Switches styles to "Welcome screen".
      */
     welcomeScreen: {
       type: Boolean,
       value: false,
       reflectToAttribute: true,
     },
   },
+
+  focus: function() {},
+
+  /**
+    * This is called from oobe_welcome when this dialog is shown.
+    */
+  show: function() {
+    var focusedElements = this.getElementsByClassName('focus-on-show');
+    if (focusedElements)
+      focusedElements[0].focus();
+
+    this.fire('show-dialog');
+  },
 });