[Sync::USS] Introduce Sync metadata tables in LoginDatabase

In order to migrate Passwords to the new USS architecture, sync
metadata should be stored together with passoword's model data in the
same database. This is to achieve atomic writes of the data and the
metadata.

This CL is introducing the tables required for storing the per-entitiy
and per-model sync metadata tables. (namely sync_entities_metadata and
sync_model_metadata respectively).

Bug: 902349
Change-Id: Ic1e5cd30880757c4ebb5e312ffe186fdadc9b25b
Reviewed-on: https://chromium-review.googlesource.com/c/1369188
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620763}
4 files changed