[Sync::USS] Deprecate ConflictResolution::USE_NEW

Almost all data types have been migrated USS and yet USE_NEW conflict
resolution has never been used.

It's now deprecated in this CL, and we should bring it back if needed.

This would unblock an upcoming clean up to remove EntityData.Clone()
method.

TBR=stevenjb@chromium.org

Bug: 953281
Change-Id: Ic5469ffafceccd90fd279242c19ed20f8b512510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577419
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653648}
18 files changed