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This patch extends SyncableServiceBasedBridge (core to pseudo-USS)
with support for encryption, via a newly introduced interface
(ModelCryptographer), which needs to be usable in the model thread
(where the bridge lives).

This should unblock the migration of PASSWORDS to pseudo-USS, because
password data must be encrypted at all times. This is a first step of
a proposal described in a dedicated section in the Design Doc:
https://docs.google.com/document/d/14ScYZ0sop921gjBwXuReIEuQJlwftqkuSM1jMK_A_x4/edit#heading=h.ev0xr4j8pkot

Actual usage of this functionality will be introduced in follow-up
patches.

Bug: 870624
Change-Id: I227d429dc952bfe1a3a4fb05cd1ab71cac9ba1c6
Reviewed-on: https://chromium-review.googlesource.com/c/1288594
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601254}