1. Never treat a cell in a nested CachedTableAccessible as a cell in the outer table. This could previously result in trying to use cache data from one CachedTableAccessible when querying another, which could cause crashes.
2. Don't create an HTMLTableRowAccessible when the <tr> has a valid ARIA role other than "row". This could previously result in a situation where <tr role="grid"> was treated as a row by its parent table, but the <tr> was also treated as a separate table.
Differential Revision: https://phabricator.services.mozilla.com/D192175