Overview
Among other things, this allows a class or mixin to define a default
role through the component's defaultState.
Some mixins come with identicial support for managing an ARIA role. Those mixins include AriaListMixin, AriaMenuMixin, DialogModalityMixin, and PopupModalityMixin. If you're using one of those mixins, you do not need to use this mixin.
API
Used by classes Backdrop, ExpandableSection, ModalBackdrop, OptionList, PlainBackdrop, PlainExpandableSection, PlainModalBackdrop, and PlainOptionList.