Introduce SyncUserSettingsMock

It's used in ProfileSyncServiceMock. This lets tests specify
expectations or default return values for user settings, which will
eventually let us get rid of the corresponding methods on
ProfileSyncService itself.

Bug: 884159
Change-Id: I58133291208a288d3766b5142da37e12e84c9baa
Reviewed-on: https://chromium-review.googlesource.com/c/1373690
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616615}
19 files changed