Refactor kiosk app launch to be part of login screen UI flow.

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 5b683c2a13a90d02942a73160d1a5239d707ad1e..5283fe742b98112d115bf0665e4662ed39637950 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
@@ -851,6 +851,10 @@ void SigninScreenHandler::RegisterMessages() {
               &SigninScreenHandler::HandleShowLoadingTimeoutError);
   AddCallback("updateOfflineLogin",
               &SigninScreenHandler::HandleUpdateOfflineLogin);
+
+  // This message is sent by the kiosk app menu, but is handled here
+  // so we can tell the delegate to launch the app.
+  AddCallback("launchKioskApp", &SigninScreenHandler::HandleLaunchKioskApp);
 }
 
 void SigninScreenHandler::RegisterPrefs(PrefRegistrySimple* registry) {
@@ -1622,6 +1626,10 @@ void SigninScreenHandler::HandleUpdateOfflineLogin(bool offline_login_active) {
   offline_login_active_ = offline_login_active;
 }
 
+void SigninScreenHandler::HandleLaunchKioskApp(const std::string& app_id) {
+  delegate_->LaunchKioskApp(app_id);
+}
+
 void SigninScreenHandler::StartClearingDnsCache() {
   if (dns_clear_task_running_ || !g_browser_process->io_thread())
     return;