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We also add the function ShouldDeprecateV1DeviceSync for future use.

We adjust the DeviceSync service unit tests to check the Chrome OS
feature flags.

Bug: 951969, 1019206
Change-Id: I5cd8de6521131cf1cca672f4dc7b6115f93cd06a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1907834Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714490}