[Sync:USS] Add integration test for Nigori initialization

Once client received default NigoriSpecifics during initial sync it
should fill NigoriSpecifics with keystore Nigori and commit changes
to the server. This CL adds integration test for this case.

Bug: 922900
Change-Id: I6bd3443fb699aeeb3609183377204a75b76fa6cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609823
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663018}
8 files changed