Properly parent transient window dialogs for the ATK API

Similar to what happens for the Windows port, the AuraLinux
accessibility implementation should walk up the transient window chain
when returning parents of dialogs in transient windows. This fixes an
issue where bubbles show up twice in the accessibility tree and where
bubbles did not return their frame parents when calling
atk_object_get_parent.

Bug: 1024342
Change-Id: I15fc2db6c6d21a7368eae92798127e597ba8b0c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919930
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719072}
6 files changed