GitLab

We're in the process of adding new PLMObservers that log UKM
metrics, so need to generalize our UKM test infra.

This change introduces the UkmTester test harness class, which
makes it easier to test logging of UKM metrics, and updates the
UKM observer to use this new class.

Other PLMObservers that will use this test infra can be seen
in https://codereview.chromium.org/2924673004

Bug: 738921
Change-Id: I84d61aca66453bcaca21f003f5e5dbfd9b6ac51c
Reviewed-on: https://chromium-review.googlesource.com/558945
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484796}