Bring brack dismiss button for kiosk toasts

The dismiss button was accidentally removed by https://crrev.com/c/3358485. The underlying API differentiates between an empty string and a nullopt.

Bug: 1309508
Test: manual
Change-Id: I2b5f46a85b1703ad80b4b49bdd7584a3147aa89a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3574565
Reviewed-by: Roman Sorokin <rsorokin@chromium.org>
Commit-Queue: Ben Franz <bfranz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#989456}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index 15097b63..67fc795 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -298,7 +298,8 @@
 void LoginScreenController::ShowKioskAppError(const std::string& message) {
   ToastData toast_data("KioskAppError", ToastCatalogName::kKioskAppError,
                        base::UTF8ToUTF16(message), ToastData::kInfiniteDuration,
-                       /*visible_on_lock_screen=*/true);
+                       /*visible_on_lock_screen=*/true,
+                       /*dismiss_text=*/std::u16string());
   Shell::Get()->toast_manager()->Show(toast_data);
 }