Migrate the iOS SyncSetupService to use SyncUserSettings

SyncUserSettings is a new class that encapsulates all the
user-configurable knobs for Sync. It replaces a bunch of setters
and getters directly on the SyncService.

Bug: 884159
Change-Id: I219dc4d37ecd4a9eac1a7fb40f50b6b0bc2cb88d
Reviewed-on: https://chromium-review.googlesource.com/c/1331402
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607577}
9 files changed