[Sync::USS] Introducing PasswordSyncBridge

This CL adds the skeleton of the PasswordSyncBridge and plumps it into
PasswordStore.
In addition it also include the feature toggle for Passwords USS
migration.

Bug: 902349
Change-Id: I343f2bb83ecf2c993bacab9e299e7d0deea868b2
Reviewed-on: https://chromium-review.googlesource.com/c/1323653
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@{#606766}
8 files changed