[cros] check for updates in WebUI OOBE can be cancelled in dev builds.

chrome/browser/resources/chromeos/login/oobe_screen_update.js

 // Copyright (c) 2011 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.
 
 /**
  * @fileoverview Oobe update screen implementation.
  */
 
 cr.define('oobe', function() {
   /**
@@ skipping 29 matching lines @@
 
     /**
      * Buttons in oobe wizard's button strip.
      * @type {array} Array of Buttons.
      */
     get buttons() {
       return null;
     }
   };
 
+  UpdateScreen.enableUpdateCancel = function() {
+    $('update-cancel-hint').hidden = false;
+    document.addEventListener('keydown', function(e) {
+      if (e.keyCode == 27) {  // Escape
+        var updateCancelHint = $('update-cancel-hint').children[0];
+        updateCancelHint.textContent =
+            localStrings.getString('cancelledUpdateMessage');
+        chrome.send('cancelUpdate');
+      }
+    });
+  };
+
   return {
     UpdateScreen: UpdateScreen
   };
 });