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A null site_for_cookies means the containing frame is already cross-site, not that
it's top-level. (And it's impossible to see from it whether it's same-site with
a resource --- for purposes of cookies everything below is third-party)

Also simplify the function to not distinguish 5 cases when it's only used
to tell 2 apart.

Bug: 1006328
Change-Id: I41d46e990fe9afe78ba502a76b85621dce74d8b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932088Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Commit-Queue: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719374}