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No behavioral changes are expected, although it's not trivial to claim
so due to subtle ordering changes.

In particular, the registration of control types is now taken care of
in SyncManagerImpl, immediately before NotifyInitializationSuccess() is
run, which continues in SyncEngineBackend where the old code lived.

Somewhat related, the mechanism to brodcast SyncEncryptionHandler-
related events via a proxy observer is also moved to SyncManagerImpl.

We do this in preparation for changing the behavior in future patches.

Bug: 922900
Change-Id: Ie43d06e40403e1bb0476ffec6f82d2596e018588
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619813
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662582}