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It's a simple test implementation of SyncUserSettings that mostly
forwards calls to the TestSyncService. It is not actually used yet, but
it unlocks the migration of tests over to SyncUserSettings.

Bug: 884159
Change-Id: Id9d9c03f639c6523bb6d6592517ba46d1f8f4566
Reviewed-on: https://chromium-review.googlesource.com/c/1335565
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608286}