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}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/public/cpp/BUILD.gn[diff]
• ash/public/cpp/login_screen.h[diff]
• ash/public/cpp/scoped_guest_button_blocker.h[Added - diff]
• ash/shelf/login_shelf_view.cc[diff]
• ash/shelf/login_shelf_view.h[diff]
• chrome/browser/chromeos/login/reset_browsertest.cc[diff]
• chrome/browser/chromeos/login/screens/reset_screen.cc[diff]
• chrome/browser/chromeos/login/screens/reset_screen.h[diff]
• chrome/browser/chromeos/login/wizard_controller.cc[diff]
• chrome/browser/ui/ash/test_login_screen.cc[diff]
• chrome/browser/ui/ash/test_login_screen.h[diff]