Function cross_origin_has_own 

pub(crate) fn cross_origin_has_own<D: DomTypes>(
    cx: &mut CurrentRealm<'_>,
    proxy: HandleObject<'_>,
    cross_origin_properties: &'static CrossOriginProperties,
    id: HandleId<'_>,
    exists: &mut bool,
) -> bool

Evaluate whether proxy has the own property ‘id’ in the cross-origin case.

cx and proxy are expected to be different-Realm here. proxy is a proxy for a maybe-cross-origin object.