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In order to allow silent migration from Directory to USS implementation
of Nigori, we need to support loading explicit passphrase key from
preferences (bootstrap_token). We just pass it to constructor of
NigoriSyncBridgeImpl and use on-demand (once we receive explicit
passphrase Nigori from the server), to avoid potentially wrong Nigori
states due to early initialization of Cryptographer.

Bug: 922900
Change-Id: I5a71f842519272f9571f201f06d3e45e8437edb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729080
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683551}