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

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

Index: chrome/browser/resources/chromeos/login/display_manager.js
diff --git a/chrome/browser/resources/chromeos/login/display_manager.js b/chrome/browser/resources/chromeos/login/display_manager.js
index b7bd9d7705268268847a985ba06276a1d8e4b900..a9b96e35e519ac6c6703f70c39354c7da8ecab09 100644
--- a/chrome/browser/resources/chromeos/login/display_manager.js
+++ b/chrome/browser/resources/chromeos/login/display_manager.js
@@ -187,6 +187,10 @@ cr.define('cr.ui.login', function() {
       } 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;