Fix sampled-in inversion for UKM events.

There may be a follow-up CL that changes the class to have
only RandInt() accessed through a virtual method and thus remove
the non-deterministic part of the test.  However, in order to merge
easily, this meat of this change is only the fix from "!=" to "=="
and a test to verify the correction.

Bug: 931710
Change-Id: Idfe0cef847148aa99b19974f29b53388d0141e2f
Reviewed-on: https://chromium-review.googlesource.com/c/1470943
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631973}
3 files changed