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Add FakeClock classes
Add unittests for:
* basic DeviceOffHoursController
* Convert*
* Apply*
* UpdateOffHoursMode
* SuspendDone observer method

Original CL written by Daria Yakovleva: https://crrev.com/c/675243

Bug: 739713

Author: Daria Yakovleva <yakovleva@google.com>
Change-Id: I35929e374d05713066027b86946f77c4c50f8473
Reviewed-on: https://chromium-review.googlesource.com/723744
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510202}