Indeed - originally UKM base::Feature was not on, so we reasoned if UKM is on, then the server config would be there and thus would specify whatever whitelist is set.

But now that it's on by default, that is no longer guaranteed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6f83c851bbefb29afd2af2acdc5ac0061509495d

commit 6f83c851bbefb29afd2af2acdc5ac0061509495d
Author: Robert Kaplow <rkaplow@chromium.org>
Date: Mon Mar 19 15:18:41 2018

Strict UKM entry whitelisting.

Currently if there is no whitelist, we record everything. We shouldn't do this since if a user has no field trial params (first run, etc.), they will record everything, avoiding the whitelist.

Also changed a bitwise and to && while I was making a change. No-op but seems bad style (and apparently can be different as & doesn't shortcut).

Bug:  820632 
Change-Id: I3fd3b1ca6669294e61467bb0f4d0ad87fb2ff576
Reviewed-on: https://chromium-review.googlesource.com/957384
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544034}
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/metrics_services_manager/metrics_services_manager.cc
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/ukm/test_ukm_recorder.cc
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/ukm/test_ukm_recorder.h
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/ukm/ukm_recorder_impl.cc
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/ukm/ukm_recorder_impl.h
[modify] https://crrev.com/6f83c851bbefb29afd2af2acdc5ac0061509495d/components/ukm/ukm_service_unittest.cc