[Sync::USS] Add transaction support to PasswordSyncStore

USS requires atomic storage of model data and sync metadata. This CL
add transaction support to the PasswordSyncStore to fulfill this
requirement.

Bug: 902349
Change-Id: I6d564d0e579d95ca08915daf802491c69019fe74
Reviewed-on: https://chromium-review.googlesource.com/c/1396034
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622450}
10 files changed