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The hypothesis here is that the test outcome is, strictly speaking,
non-deterministic, because a server with an eventual consistency model
cannot guarantee how exactly a conflict will be resolved (although it
will guarantee that clients converge). In this model, the last commit
will win.

We fix the previously existing test by making sure which of the two
clients commits first to the server. This is achieved by mimic-ing an
offline state (or poor connectivity compared to the other client).

In order to verify the hypothesis, we add a second test that enables
strong consistency model on the server (implemented as part of this
patch), and hence relies on that mechanism to expect a deterministic
outcome without having to put clients offline.

Bug: 810408
Change-Id: I2fa3fd8c941674344030a76e76f7855131c1908e
Reviewed-on: https://chromium-review.googlesource.com/1249482
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594865}