Fix OWNERship of c/b/chromeos/login/signin/*signin_error_notifier*

- Point *signin_error_notifier*'s per-file OWNERship to OWNERS of Chrome
  OS Account Manager.
- Add anastasiian@ as an OWNER of Chrome OS Account Manager.

Change-Id: Ice2e9d8231c6f8cba2f5f6aad408425478b0550f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214944Reviewed-by: Anastasiia N <anastasiian@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771548}

chrome/browser/chromeos/login/signin/OWNERS

+per-file *signin_error_notifier*=file://chromeos/components/account_manager/OWNERS

chromeos/components/account_manager/OWNERS

 sinhak@chromium.org
+anastasiian@chromium.org