Elements in internal shadow trees that represent CSS pseudo-elements
(e.g. the DetailsContent slot in <details>) store their cascaded
properties on the host element's pseudo-element data, not on the
element itself. This meant that when slotted elements walked the
inheritance chain and encountered such a slot, they would dereference
null cascaded properties and crash.
Fix this by copying the cascaded properties onto the slot element
itself after computing its style, keeping both cascaded and computed
properties accessible in the same place.
