[profiles] Allow ProfilePickerHandler to unregister the guest profile

The profile attributes for the ephemeral guest profile aren't initially
marked as guest. ProfileAttributesEntry::SetIsGuest() is called after
ProfilePickerHandler::OnProfileAdded() gets notified. This means that
ProfilePickerHandler registers the guest profile in the profiles_order_
list. ProfilePickerHandler starts ignoring the ephemeral guest profile
after it's marked as guest in storage which creates conflict with the
profile_order_'s state.

OnProfileIsOmittedChanged() can be used to remove the ephemeral guest
profile from profile_order_ when the profile is marked as omitted.
Guest profiles are currently ignored in that method.

This CL allows to remove ephemeral guest profiles from profiles_order_
when OnProfileIsOmittedChanged() is called.

Bug: 1201745
Change-Id: Ie70c33db390cd85b6e17b1c9cb0e5c3b5908267f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2846519
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#875708}