Layout doesn't consider these types of th elements to be table cells since there
is no underlying table layout. This patch asks layout whether it considers the
frame's accessible type to be a table cell accessible. If it does, continue to
use the HTMLTableHeaderCellAccessible, but - otherwise - use a generic grid cell
accessible. This revision also adds a test to verify the behavior of th in this
situation. This essentially repeats the logic already in existence for td.
Differential Revision: https://phabricator.services.mozilla.com/D162357
af71bdeca8