Add OWNERS for chrome.loginScreenUi API

This CL adds hendrich@chromium.org and voit@google.com as OWNERS for
the loginScreenUi API.

Bug: 957573
Change-Id: I367b6d7111ba5454ee4ee14505c8500adcc20c33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645293Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666606}

chrome/browser/chromeos/extensions/login_screen_ui/OWNERS

+hendrich@chromium.org
+voit@google.com