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

chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc

Index: chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
index cb12e5647b3e821ea024c69196c292109ca06c81..26f1efc13f43cfcfaea76146cd4c4628053384bc 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
@@ -496,6 +496,9 @@ void SigninScreenHandler::RegisterMessages() {
   web_ui()->RegisterMessageCallback("openProxySettings",
       base::Bind(&SigninScreenHandler::HandleOpenProxySettings,
                  base::Unretained(this)));
+  web_ui()->RegisterMessageCallback("loginVisible",
+      base::Bind(&SigninScreenHandler::HandleLoginVisible,
+                 base::Unretained(this)));
 }
 
 void SigninScreenHandler::HandleGetUsers(const base::ListValue* args) {
@@ -1015,6 +1018,13 @@ void SigninScreenHandler::HandleOpenProxySettings(const base::ListValue* args) {
   BaseLoginDisplayHost::default_host()->OpenProxySettings();
 }
 
+void SigninScreenHandler::HandleLoginVisible(const base::ListValue* args) {
+  content::NotificationService::current()->Notify(
+      chrome::NOTIFICATION_LOGIN_WEBUI_VISIBLE,
+      content::NotificationService::AllSources(),
+      content::NotificationService::NoDetails());
+}
+
 void SigninScreenHandler::StartClearingDnsCache() {
   if (dns_clear_task_running_ || !g_browser_process->io_thread())
     return;