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

chrome/browser/resources/chromeos/login/display_manager.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 Display manager for WebUI OOBE and login.
  */
 
 // TODO(xiyuan): Find a better to share those constants.
 /** @const */ var SCREEN_OOBE_NETWORK = 'connect';
@@ skipping 169 matching lines @@
           oldStep.removeEventListener('webkitTransitionEnd', f);
           if (oldStep.classList.contains('faded') ||
               oldStep.classList.contains('left') ||
               oldStep.classList.contains('right')) {
             oldStep.classList.add('hidden');
           }
         });
       } else {
         // First screen on OOBE launch.
         newHeader.classList.remove('right');
+        // Report back first OOBE screen being painted.
+        window.webkitRequestAnimationFrame(function() {
+          chrome.send('loginVisible');
+        });
       }
       this.currentStep_ = nextStepIndex;
       $('oobe').className = nextStepId;
     },
 
     /**
      * Show screen of given screen id.
      * @param {Object} screen Screen params dict,
      *     e.g. {id: screenId, data: data}.
      */
@@ skipping 223 matching lines @@
         });
       }
     }
   };
 
   // Export
   return {
     DisplayManager: DisplayManager
   };
 });