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Nigori is a special datatype because failing to commit local changes
(e.g. custom passphrase set) can lead to problematic scenarios (e.g.
user data exposed unencrypted) or in rare cases complicated
conflict-resolution scenarios (e.g. keystore rotation took place before
committing the custom passphrase) that worst-case cannot be resolved
synchronously (e.g. keystore keys are missing and hence keys are
pending for some time, until keystore keys are received).

It's very hard to universally reason about all side effects of local
commits and conflict resolution cases. Instead, this patch introduces
infrastructure that adopts the command design pattern to concisely
represent what the intended local change is.

Local changes may now fail to apply, in particular if there is a
conflict. But in such case the infrastructure allows dealing with the
failure, with custom logic that depends on the precise command type.

Similarly, success is only reported once the commit is acked by the
sync server, which is a desirable property and useful foundation for
future work. This may introduce additional latency in the existing UX,
e.g. while setting up a custom passphrase, but manual tests indicate
that it's not noticeable.

In this patch, only the infrastructure is introduced without actually
adopting it for the various local changes. Future patches will take
care of such adoption.

Bug: 922900
Change-Id: Ib55d3a401f1f68a1a910ee1e2b9119889d4f73b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864770
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#706401}