LibWeb: Update ARIA default-role handling for img, section, and aside

[sideshowbarker]

This change updates our handling of computation for default ARIA roles for the img, section, and aside elements to match the expectations in the updated WPT test at http://wpt.live/html-aam/roles-contextual.html.

This gets us (re)passing all but one of the (updated) subtests in the test areturn://wpt.fyi/results/html-aam/roles-contextual.html

But it’s not possible to pass that subtest without regressing our (passing) behavior for the test at https://wpt.fyi/results/accname/name/comp_tooltip.tentative.html

So I’ve raised https://github.com/w3c/aria/issues/2491 to ask the ARIA working group and the spec editors to resolve the spec conflict.