[tab-under] Add UMA for engagement score

This will help us see how much we stand to lose by relaxing tab-under
protection to sites with low (or 0) site engagement scores.

A followup will probably land a relaxed blocking scheme for origins
with nonzero engagement, possibly behind another variation flag.
This data will give us better insight into whether our hypothesis
is right: namely, that tab-under destinations will often have low
engagement.

See doc for tab-under relaxation:
https://docs.google.com/document/d/1N-0etmATN6PhQt-m9S8sCgfz88zv8lZ2dBFdqKlzsPg/edit?ts=5aa73bff#

Bug: 661629
Change-Id: I535710fdde0a162c53994b0df99f6333a8af99ce
Reviewed-on: https://chromium-review.googlesource.com/959538
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543161}
5 files changed