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1. Unref AtkStateSet and AtkRelationSet when we're finished with them.

2. For atk_table_get_column_description and _row_description, we're
dynamically generating a value but the API appears to expect us to
return a static string. For now I've scoped the existing LSAN disabling
down to include just this spot since all remaining leaks are fixed.

Bug: 568674
Change-Id: I7174312452ae118eacb706cd7e686bf8649a790f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688317
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Cr-Commit-Position: refs/heads/master@{#676103}