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If the experimental feature for Nigori USS is enabled, the new codepath
now uses a controller (ModelTypeController) to start and stop Nigori,
and to connect it to the ModelTypeWorker.

Because of lifetime issues with Nigori, and the special treatment for
the initial sync cycle during engine initialization (which predates the
instantiation of DataTypeManager), the interactions with the controller
are done manually, and on the sync thread. This is not ideal, but we
don't have better alternatives as of today, without intrusive changes
that would be barely feasible during the coexistence of both codepaths.

To avoid engine_impl's dependence on driver the registration of Nigori
datatype was moved back to SyncEngineBackend (partially revert
https://chromium-review.googlesource.com/c/chromium/src/+/1619813).

Bug: 922900
Change-Id: I22b20d3a0e2f396a0eddbef54cf2e429f02cc410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648178
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667114}