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The newly introduced mechanism around PendingLocalNigoriCommit allows
implementing NigoriSyncBridgeImpl::SetEncryptionPassphrase(),
responsible for setting up a custom passphrase, in a way that:
a) Success is only reported when the sync server acks the commit.
b) Conflict-resolution becomes simple.
c) Logic is factored out from NigoriSyncBridgeImpl.

There is a risk that the round trip to the sync server introduces
noticeable UX latency when setting up a custom passphrase. However,
manual tests suggests that it's actually not noticeable.

Bug: 922900
Change-Id: I76f6644ed313d05aca8fe1d7cdf1d3205c7282cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856968
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#706452}