Add ProfileHelper::SigninProfileHasLoginScreenExtensions()

This CL adds a helper method to ProfileHelper to check whether the
sign-in profile has force-installed login screen extensions. This
method is used in a follow-up CL to determine whether the extension
cache should be created with a higher task priority.

Bug: none
Change-Id: If9b2a2204c68a484217024d0ab549b2fc1bef837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678499
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672824}
3 files changed