Add initial version of UserPolicyMixin

For now, the mixin provides utility for setting up cached policy blob in
session manager client, but more functionality will likely be added in
future.

Use the mixin in CrashRestoreChildUserTest and EncryptionMigrationTest,
both of which set up a cached policy.

BUG=959244

Change-Id: I9bfe194ab2aecbd0039dd7ca5c6cca9e0d62ef33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589670
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656853}
5 files changed