[tab-under] Gate via Site Engagement

This CL adds a variation parameter which encodes a threshold for
the Site Engagement score [1] required to do tab-unders. By default,
we allow tab-unders to origins that have any non-zero engagement.

This moves the Tab.TabUnder.EngagementScore metric as the last check
of IsSuspiciousClientRedirect, to simulate the scores blocked tab-unders
would have if we removed that check.

[1]: https://www.chromium.org/developers/design-documents/site-engagement

Bug: 661629
Change-Id: I48ba83ab8cc58641cc21dcad062c313f1d1290ef
Reviewed-on: https://chromium-review.googlesource.com/965403
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543779}
3 files changed