Portals: Prevent closing of the portal window.

Bug: 1110926
Change-Id: Ib22e96d4898f08f326094bfdc030eb7a73b65e00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353016


Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798125}