CustomStateSet directly selected the style invalidation reason used when
its JS-visible set is modified. Move that mapping into
CSS::Invalidation::CustomElementInvalidator.
This keeps the custom-state container focused on its set contents while
CSS invalidation owns the style work required by :state() selectors.
Element.cpp still spelled out the :defined pseudo-class invalidation set
when custom element state changed. Move that selector policy into
CustomElementInvalidator.
This keeps Element responsible for the state transition, while
CSS::Invalidation owns the affected selector feature.