ChromeOS OOBE: fix focus flow in MD-OOBE.

chrome/browser/resources/chromeos/login/oobe_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">
 
 <!--
   Simple OOBE dialog which should be used for OOBE UI elements.
   It has correct size and padding. It can display top left icon, and has
   three parts: header (which should have <h1>title</h1>), footer (content part),
   and optional buttons container at the bottom.
 
+  When shown (i.e. when outside container calls .show()):
+    1. If this dialog has tags in class "focus-on-show", the first one will be
+  focused.
+    2. 'show-dialog' is fired.
+
   Example:
     <link rel="stylesheet" href="oobe_dialog_parameters.css">
-    <oobe-dialog has-buttons>
+    <oobe-dialog on-show-dialog="onTestDialogShown_" has-buttons>
       <iron-icon ... class="oobe-icon">
       <div class="header">
         <h1 class="title">Title</h1>
         <div class="subtitle">Subtitle</div>
       </div>
       <div class="footer">
+        <div class="focus-on-show">...</div>
         ...
       </div>
       <div class="bottom-buttons">
         ...
       </div>
     </oobe-dialog>
 
   Add class |header| to all which you want to go inside the header.  Similar
   with clases |footer|, |bottom-buttons| and |oobe-icon|. |bottom-buttons| block
   is shown only if |has-buttons| attribute is set.
@@ skipping 14 matching lines @@
       <content select=".footer"></content>
     </div>
     <template is="dom-if" if="[[hasButtons]]">
       <div id="oobe-bottom" class="layout horizontal center">
         <content select=".bottom-buttons"></content>
       </div>
     </template>
   </template>
 </dom-module>