According to SVG Accessibility API Mappings, an SVG element which has title or desc element must be exposed.
https://w3c.github.io/svg-aam/#include_elements
Previously, we didn't expose <g> elements at all even if they had a title/desc, and we unconditionally exposed some other SVG elements even when they didn't.
This removes the Dev Tools A11y Panel code which explicitly allowed unlabelled descendants of role="img" <svg> elements, since we don't create descendants if they don't have a label now anyway.
The associated tests had to be tweaked as well, since now we don't create unlabelled descendants.
Original patch by Takeshi Kurosawa.
Differential Revision: https://phabricator.services.mozilla.com/D77763