login_manager: Refine PolicyDescriptor proto

The account type (device, user, ...) and the policy domain (Chrome,
extensions) in PolicyDescriptor are in fact two independent dimensions.
The current PolicyDescriptor does not reflect that. For instance,
storing extension policy for device local accounts wouldn't be
possible, but it's a valid use case. This CL fixes that.

Note that the CL isn't used on the Chrome side yet, so it's safe to
change.

CQ-DEPEND=CL:730223

BUG=chromium:765644
TEST=emerge-amd64-generic system_api

Change-Id: I348019b665ba12584e988745330d6d10dc263b76
Reviewed-on: https://chromium-review.googlesource.com/730183
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
1 file changed