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We used to ask Directory itself to provide Nigori node for debugging.
With USS implementation enabled we need to change the source to
corresponding ModelTypeController. Since Nigori's ModelTypeController
not available inside ProfileSyncService, we plumb it through SyncEngine
and SyncEngineBackend.

There is a side change: when Directory implementation of Nigori is
enabled, we start call GetAllNodesForTypeFromDirectory() from sync
sequence, instead of UI sequence. It should be safe, since logging
already supports plumbing to different sequences and Directory's
functions could be called from any sequence (protected by
transactions).

Bug: 922900
Change-Id: Ifdc5ffb1a5a99e51a0ec88643f9325d1c4a287de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800772
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701127}