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

chrome/common/chrome_notification_types.h

Index: chrome/common/chrome_notification_types.h
diff --git a/chrome/common/chrome_notification_types.h b/chrome/common/chrome_notification_types.h
index cf151dac83743f818aa5940a9879d13d99aa2726..ef28626d2e11f4eae680695e6b93b9480e64ff62 100644
--- a/chrome/common/chrome_notification_types.h
+++ b/chrome/common/chrome_notification_types.h
@@ -891,6 +891,14 @@ enum NotificationType {
   // Sent when the user images on the WebUI login screen have all been loaded.
   NOTIFICATION_LOGIN_USER_IMAGES_LOADED,
 
+  // Sent when the login WebUI is considered to be fully visible.
+  // That momemnt is tracked as the first paint event after one of the:
+  // 1. NOTIFICATION_LOGIN_USER_IMAGES_LOADED
+  // 2. NOTIFICATION_LOGIN_WEBUI_READY
+  // 3. NOTIFICATION_LOGIN_NETWORK_ERROR_SHOWN
+  // 4. NOTIFICATION_WIZARD_FIRST_SCREEN_SHOWN
+  NOTIFICATION_LOGIN_WEBUI_VISIBLE,
+
   // Sent when proxy dialog is closed.
   NOTIFICATION_LOGIN_PROXY_CHANGED,