ChromeOS: Remove Guest button when Reset screen is displayed.

This Cl adds ScopedGuestButtonBlocker to temporarily hide Guest login button,
and blocks this button while Reset screen is shown.


Bug: 962398
TBR=oshima@chromium.org

Change-Id: I799d5723b4aea2a999a383c8635c790a2f510047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672629
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680731}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index 0a87a9ffe..f75b7b1 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -364,6 +364,14 @@
       ->SetAllowLoginAsGuest(allow_guest);
 }
 
+std::unique_ptr<ScopedGuestButtonBlocker>
+LoginScreenController::GetScopedGuestButtonBlocker() {
+  return Shelf::ForWindow(Shell::Get()->GetPrimaryRootWindow())
+      ->shelf_widget()
+      ->login_shelf_view()
+      ->GetScopedGuestButtonBlocker();
+}
+
 void LoginScreenController::ShowLockScreen() {
   OnShow();
   LockScreen::Show(LockScreen::ScreenType::kLock);