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This replaces a custom subclass of FakeSyncService with the standard
syncer::TestSyncService. It also removed another custom subclass that's
just not used at all.

Bug: 859874
Change-Id: I0ef20ca70a1fdc81649c856c287ed6f1432956f8
Reviewed-on: https://chromium-review.googlesource.com/c/1326149Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606445}