[tab-under] Relax suspicion to cross-site rather than cross-origin

This partially fixes breakage to gmail, which wants to redirect
mail.google.com -> accounts.google.com for auto-logout.

See the design doc for relaxing tab-under protection:
https://docs.google.com/document/d/1N-0etmATN6PhQt-m9S8sCgfz88zv8lZ2dBFdqKlzsPg/edit?ts=5aa2f94e#

It feels like this change is the least controversial and it won't reduce the
efficacy of the intervention.

Bug: 661629
Change-Id: I01c7435a12515aacf011a722ee155d60cb1d6bae
Reviewed-on: https://chromium-review.googlesource.com/959689
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542835}
3 files changed