Lookalikes: Ignore nonunique domains

This CL filters out nonunique domains so that navigations to such
domains don't record metrics or show an interstitial. It also
prevents such sites from being added to the engaged sites list.

Bug: 951509
Change-Id: Iaaa720b40d967d75d3ced48e20d4dbaa7c0308fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566378
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651377}