login: Call completion callback on LoginDisplayHost destruction

This CL lets LoginDisplayHost calls completion_callbacks_ added from
Finalize or StartUserAdding immediately on destruction for all derived
classes by followings:
* {Fake, Mock}LoginDisplayHost calls completion_callbacks_.

We should call completion_callback_ immediately instead of posting to
SingleThreadTaskRunner for LoginDisplayHostCommon, but we need to fix
UserAddingScreen ownership beforehand which will be done in the next cl.

For now, UserAddingScreen is never destroyed, so calling
completion_callbacks_ asynchrounsly does not cause a memory leak.

This CL also changes user_adding_screen.cc to resolve IWYU.

Bug: 1469521
Change-Id: Ibc158ad57700abc25a05ff37fb2229d11e4dd9fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4778028
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Eriko Kurimoto <elkurin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1191935}