[cros] Change how we track momemt to emit login-prompt-visible signal.

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

 // Copyright (c) 2012 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 User pod row implementation.
  */
 
 cr.define('login', function() {
   /**
@@ skipping 848 matching lines @@
      */
     handlePodImageLoad: function(pod) {
       var index = this.podsWithPendingImages_.indexOf(pod);
       if (index == -1) {
         return;
       }
 
       this.podsWithPendingImages_.splice(index, 1);
       if (this.podsWithPendingImages_.length == 0) {
         chrome.send('userImagesLoaded');
+        // Report back user pods being painted.
+        window.webkitRequestAnimationFrame(function() {
+          chrome.send('loginVisible');
+        });
       }
     }
   };
 
   return {
     PodRow: PodRow
   };
 });