Fix flaky NPE in oobe.js.

chrome/browser/resources/chromeos/login/login_shared.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.
 
 /**
  * @fileoverview Common OOBE controller methods. This method is shared between
  * OOBE, login, and the lock screen. Add only methods that need to be shared
  * between all *three* screens here, as each additional method increases the
  * time it takes to show the lock screen.
  *
@@ skipping 262 matching lines @@
   };
 
   /**
    * Restores input focus to currently selected pod.
    */
   Oobe.refocusCurrentPod = function() {
     DisplayManager.refocusCurrentPod();
   };
 
   /**
+   * Some ForTesting APIs directly access to DOM. Because this script is loaded
+   * in header, DOM tree may not be available at beginning.
+   * In DOMContentLoaded, this is marked to true, indicating ForTesting methods
+   * can be called.
+   * External script using ForTesting APIs should wait for this condition.
+   * @type {boolean}
+   */
+  Oobe.readyForTesting = false;
+
+  /**
    * Skip to login screen for telemetry.
    */
   Oobe.skipToLoginForTesting = function() {
     Oobe.disableSigninUI();
     chrome.send('skipToLoginForTesting');
   };
 
   /**
    * Login for telemetry.
    * @param {string} username Login username.
@@ skipping 140 matching lines @@
          src instanceof HTMLInputElement &&
          /text|password|search/.test(src.type);
 });
 
 
 (function() {
   'use strict';
 
   document.addEventListener('DOMContentLoaded', function() {
     Oobe.initialize();
+    Oobe.readyForTesting = true;
   });
 
   // Install a global error handler so stack traces are included in logs.
   window.onerror = function(message, file, line, column, error) {
     console.error(error.stack);
   }
 })();
-