[Sync::USS] Expose the MetadataStore in PasswordStoreSync interface

This CL exposes the MetadataStore in PasswordStoreSync interface.
This will be useful later because the PasswordSyncBridge will require
a MetadataStore to persist the metadata.

Bug: 902349
Change-Id: I603299846828246a23822ca3a39126c3b4de0f61
Reviewed-on: https://chromium-review.googlesource.com/c/1401047
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@{#622594}
6 files changed