[Sync::USS] Fix for privacy with USS Sync Password metadata persistence

Before this CL:
USS Sync stores a serilaized sync metadata proto, which contains a hash
of the specifics that contains the password in plain text.

After this CL:
Sync uses the same mechanism to encrypt password values to encrypt
sync metadata.

TBR=mastiz@chromium.org

Bug: 941988
Change-Id: Ib5c8d27e1f5beecb1e5c7fa9c1a5fe70ed765b14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1523097
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641231}
3 files changed