[Sync::USS] LoginDatabase should implement syncer::SyncMetadataStore

USS bridge require a sync metadata store to read and store the sync
LoginDatabase is the best fit for this for Passwords data type.
This CL adds implements syncer::SyncMetadataStore in

Bug: 902349
Change-Id: I1eef9b5aa3bbf99f8d7c5ad1fc20884781ca257b
Reviewed-on: https://chromium-review.googlesource.com/c/1371901
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620799}
4 files changed