Implement AtkTable for ARIA treegrids

AtkTable was supported for all objects of ATK_ROLE_TABLE. Because ARIA
treegrid is mapped to ATK_ROLE_TREE_TABLE, this support was absent.

Bug: 978056
Change-Id: I39caa49e53f4864fd16f0eab5e4164f5fb8f9ee1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672747
Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Commit-Queue: Joanmarie Diggs <jdiggs@igalia.com>
Cr-Commit-Position: refs/heads/master@{#672061}
4 files changed