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Currently, we're setting the "site for cookies" only by looking at the
top-level document. We ought to be verifying that the ancestor frames
are same-site before doing so. We do this correctly in Blink (see
`Document::SiteForCookies`), but didn't do so when navigating in the
browser.

This patch addresses the majority of the problem by walking the ancestor
chain when processing a NavigationRequest. If all the ancestors are
same-site, we set the "site for cookies" to the top-level document's URL.
If they aren't all same-site, we set it to an empty URL to ensure that
we don't send SameSite cookies.

Bug: 833847
Change-Id: Icd77f31fa618fa9f8b59fc3b15e1bed6ee05aabd
Reviewed-on: https://chromium-review.googlesource.com/1025772Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553942}