Use Registrar in OsRegistration

This makes it clear that the client cannot be disabled for OS
registrations.

Change-Id: I3249530b8139ff13abb3d330b69311dbce9656a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5414814


Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282291}