SyncUserSettings: Read/write FirstSetupComplete directly from/to prefs

Effectively, this moves the source of truth for this bit from
ProfileSyncService to SyncUserSettings, where it belongs.
ProfileSyncService now listens for changes to the pref value and reacts
accordingly.

Bug: 884159
Change-Id: Id8a8429aecf8854438ead8fbbd32ac6a26e951b6
Reviewed-on: https://chromium-review.googlesource.com/c/1355120
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617068}
11 files changed