Fix flaky NPE in oobe.js.

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

Index: chrome/browser/resources/chromeos/login/login_shared.js
diff --git a/chrome/browser/resources/chromeos/login/login_shared.js b/chrome/browser/resources/chromeos/login/login_shared.js
index 314c671eb3a639b26024e92e530807b18cae6116..00832b5b8724636ad562c8528fcaca1d92a2e40b 100644
--- a/chrome/browser/resources/chromeos/login/login_shared.js
+++ b/chrome/browser/resources/chromeos/login/login_shared.js
@@ -280,6 +280,16 @@ cr.define('cr.ui', function() {
   };
 
   /**
+   * 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() {
@@ -440,6 +450,7 @@ disableTextSelectAndDrag(function(e) {
 
   document.addEventListener('DOMContentLoaded', function() {
     Oobe.initialize();
+    Oobe.readyForTesting = true;
   });
 
   // Install a global error handler so stack traces are included in logs.
@@ -447,4 +458,3 @@ disableTextSelectAndDrag(function(e) {
     console.error(error.stack);
   }
 })();
-