[A11y] Add term role to name calculation

Previously, the term role was excluded from the name from contents part
of the accessible name computation process.

This change adds a name to <dfn> elements and elements with role="term".
Additionally, content within <dfn> elements or elements with role="term"
is included in the name for parent elements.

Bug: 381418589
Change-Id: Id9ff619f6a8cd5b0d0202749de24f94c2b9a2d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6072056
Commit-Queue: Jocelyn Tran <jocelyntran@google.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1394433}