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..5e2561f647d76f1c1de2f0d51a687ef38cb0308d 100644
--- a/chrome/browser/resources/chromeos/login/login_shared.js
+++ b/chrome/browser/resources/chromeos/login/login_shared.js
@@ -440,6 +440,12 @@ disableTextSelectAndDrag(function(e) {
 
   document.addEventListener('DOMContentLoaded', function() {
     Oobe.initialize();
+
+    // Some ForTesting APIs directly access to DOM. Because this script
+    // is loaded in header, DOM tree may not be available at beginning.
+    // initialize() is called on DOMContentLoaded, so here mark ready,
+    // and let the external script wait for this condition.
+    Oobe.readyForTesting = true;
   });
 
   // Install a global error handler so stack traces are included in logs.
@@ -447,4 +453,3 @@ disableTextSelectAndDrag(function(e) {
     console.error(error.stack);
   }
 })();
-