Portals: Disallow Portals inside nested browsing contexts

Bug: 938634
Change-Id: Id9c46473a484531b5280c407e503948a2593f45f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504298
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639176}
4 files changed