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There are two types of cros settings:
1. Device settings that are read by DeviceSettingsProvider and can
be written by OwnerSettingsService
2. kSystemTimezone, which is the only system setting (or at least,
the only writeable one). It is read by SystemSettingsProvider and
can be written by TimezoneSettings.

Both of these can also be written by CrosSettings, which delegates
to either OwnerSettingsService or to TimezoneSettings.
The write path is being removed from CrosSettings, and so must
be replaced by calls to either OwnerSettingsService or
TimezoneSettings.

This CL special cases kSystemTimezone in prefs_util.cc so that
the remaining call to CrosSettings can be replaced with a call
to OwnerSettingsService.

Bug: 433840
Change-Id: Ie3d954980dbffc9c850f5afa82bedf633c01f97f
Reviewed-on: https://chromium-review.googlesource.com/c/1339940Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: A Olsen <olsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609239}