Add ATK popup-for accessibility relation

This relation is used to map from a popup back to its owning element.
Orca needs this in order to properly walk up the accessibility tree to
find the parent frame of the popup.

Bug: 948787
Change-Id: I88587be59149ea667dc071f699ea3b68814f0516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556786
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652574}
9 files changed