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..def1ebd67a78fa19a92c2ac3153946415edfd7da 100644
--- a/chrome/browser/resources/chromeos/login/login_shared.js
+++ b/chrome/browser/resources/chromeos/login/login_shared.js
@@ -52,6 +52,16 @@ 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;
+
+  /**
    * Handle accelerators. These are passed from native code instead of a JS
    * event handler in order to make sure that embedded iframes cannot swallow
    * them.
@@ -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);
   }
 })();
-