Move source of truth for SetSyncRequested to SyncUserSettings

This also lets us remove ProfileSyncService::RequestStart/Stop.
(RequestStop is now identical to StopImpl, and RequestStart can be
easily inlined at the call sites.)

Bug: 884159
Change-Id: I03ca6040e444d351a5fe554adba0f0e685abc768
Reviewed-on: https://chromium-review.googlesource.com/c/1412031
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623241}
6 files changed