When a sheet's rightmost compound carries :first-child, :last-child,
or :only-child but no other class/tag/id/attr feature, the sheet
add/remove path can target the boundary children directly instead of
falling back to a whole-subtree invalidation.
The narrowing in extend_style_sheet_invalidation_set_with_style_rule
deliberately only kicks in when the structural pseudo is the only
feature in the rightmost compound. A selector like `.foo:first-child`
keeps targeting `.foo` rather than every first-child in the
document.
Adds matchers for :first-child / :last-child / :only-child in
InvalidationSetMatcher so the targeted walk can recognize them on
each candidate element.
98a9c36ba7