Portals: Add a WindowEventHandler to the PortalActivateEvent.

Bug: 914122
Change-Id: Ie1817a21216d86f59a708accea43e1d1ef316d6c
Reviewed-on: https://chromium-review.googlesource.com/c/1412794
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623445}
13 files changed