Implements frame sniffer to watch iframe loading state.

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

Index: chrome/browser/resources/chromeos/login/oobe.js
diff --git a/chrome/browser/resources/chromeos/login/oobe.js b/chrome/browser/resources/chromeos/login/oobe.js
index e29b2faf38059f6945ef34cf35861c950ebc263c..27616b77fca30732db8c897705d1c6fe6cc0c5ab 100644
--- a/chrome/browser/resources/chromeos/login/oobe.js
+++ b/chrome/browser/resources/chromeos/login/oobe.js
@@ -136,7 +136,8 @@ cr.define('cr.ui', function() {
       // Adjust inner container height based on new step's height.
       $('inner-container').style.height = newStep.offsetHeight + 'px';
 
-      if (this.currentStep_ != nextStepIndex) {
+      if (this.currentStep_ != nextStepIndex &&
+          !oldStep.classList.contains('hidden')) {
         oldStep.addEventListener('webkitTransitionEnd', function f(e) {
           oldStep.removeEventListener('webkitTransitionEnd', f);
           oldStep.classList.add('hidden');
@@ -224,7 +225,6 @@ cr.define('cr.ui', function() {
     prepareForLoginDisplay_ : function() {
       for (var i = 0, screenId; screenId = this.screens_[i]; ++i) {
         var screen = $(screenId);
-
         screen.classList.add('faded');
         screen.classList.remove('right');
         screen.classList.remove('left');