Wait for the battery level before starting encryption migration.

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

 <!-- Copyright 2017 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-progress/paper-progress.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 <link rel="import" href="chrome://oobe/custom_elements.html">
 
 <dom-module id="encryption-migration">
   <template>
     <link rel="stylesheet" href="encryption_migration.css">
     <link rel="stylesheet" href="oobe_dialog_parameters.css">
     <template is="dom-if" if="[[isInitial_(uiState)]]">
       <oobe-dialog tabindex="0" has-buttons>
         <p>Checking the system...</p>
       </oobe-dialog>
     </template>
     <template is="dom-if" if="[[isReady_(uiState)]]">
       <oobe-dialog tabindex="0" has-buttons>
         <div class="header">
           <h1 class="title">Your file system needs upgrade</h1>
           <div class="subtitle">
             Your file system needs to be upgraded to use Android applications.
             String TBD.
           </div>
         </div>
-        <div class="bottom-buttons flex layout horizontal">
-          <div class="flex"></div>
-          <oobe-text-button on-tap="onSkip_">Skip</oobe-text-button>
-          <oobe-text-button inverse on-tap="onUpgrade_">Upgrade
-          </oobe-text-button>
-        </div>
+        <template is="dom-if" if="[[!isEnoughBattery]]">
+          <div class="footer flex layout vertical">
+            <div>Battery too low for update. <span>[[batteryPercent]]</span>%
+            </div>
+            <div>Please plug your Chromebook into a power source.</div>
+          </div>
+        </template>
+        <template is="dom-if" if="[[!isResuming]]">
+          <div class="bottom-buttons flex layout horizontal">
+            <div class="flex"></div>
+            <oobe-text-button on-tap="onSkip_">Skip</oobe-text-button>
+            <oobe-text-button inverse on-tap="onUpgrade_"
+                disabled="[[isMigrationAccepted]]">Upgrade</oobe-text-button>
+          </div>
+        </template>
       </oobe-dialog>
     </template>
     <template is="dom-if" if="[[isMigrating_(uiState)]]">
       <oobe-dialog tabindex="0">
         <div class="header">
           <h1 class="title">Upgrading...</h1>
           <div class="subtitle">Please do not shutdown. String TBD.</div>
         </div>
         <div class="footer flex layout vertical">
           <paper-progress value="[[progress]]" max="1" step="0.001"
@@ skipping 48 matching lines @@
           </template>
           <template is="dom-if" if="[[isResuming]]">
             <oobe-text-button inverse on-tap="onRestart_">Restart
             </oobe-text-button>
           </template>
         </div>
       </oobe-dialog>
     </template>
   </template>
 </dom-module>