GitLab

- Changes OwnerSettingsServiceChromeOsFactory so that its stubbed
behavior can be used without setting a switch
- Changes CrosSettings so that it can be stubbed during a test without
worrying about it being reinitialized later
- Add a new class ScopedTestingCrosSettings that makes use of this
new functionality, is much simpler than ScopedCrosSettingsTestHelper.
- Changes one test - users_private_apitest.cc - that previously used
switches, so that it no longer uses switches, as a proof-of-concept.

For more details, see the bug.

Bug: 909635
Change-Id: I57b053038592512fb7cfb7c09061d5d03475e979
Reviewed-on: https://chromium-review.googlesource.com/c/1392186
Commit-Queue: A Olsen <olsen@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619620}