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This removes complexity from the bridge to a struct with much simpler
semantics, and allows future patches to compute specifics from
NigoriState instances other than the authoritative copy owned by the
bridge.

Bug: 922900
Change-Id: I0dd60f316145700502e22a1dbc1460b3e1bcf703
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862457
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#706084}