cros: Prevent casual shut down and power save during migration.

chrome/browser/resources/chromeos/login/header_bar.css

 /* Copyright (c) 2012 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. */
 
 #login-header-bar {
   -webkit-padding-start: 15px;
   bottom: 0;
   left: 0;
+  min-height: 34px;  /* Should be consistent with .header-bar-item's height. */
   padding-bottom: 6px;
   position: absolute;
   right: 0;
 }
 
 #login-header-bar {
   background-color: rgba(0, 0, 0, 0.6);
   padding-top: 7px;
 }
 
@@ skipping 140 matching lines @@
   background-image: -webkit-image-set(
       url(images/1x/signout.svg) 1x,
       url(images/2x/signout.svg) 2x );
 }
 
 .button-restricted {
   border: 1px solid transparent;
   color: white !important;
   opacity: 0.4 !important;
 }