Device policy to allow powerwash

This is a device policy that specifies if the device is allowed to
powerwash, either from login screen with shortcut keys, or from settings
page.

The implementation of the effects on the settings page will be done in a
separate CL.

Bug: chromium:900369
Test: unit_tests --gtest_filter=DeviceSettingsProviderTest.DevicePowerwashAllowed
Change-Id: I3d70c18c6053dbe80d83213dcee543f0b2a96230
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599619
Commit-Queue: Igor <igorcov@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Igor <igorcov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672062}
10 files changed