Review utility of CookieDomain::NotPresent variant

[pfernie]

This variant was originally intended to disambiguate between an empty string Domain: value being specified (undefined behavior) and no Domain attribute being present on the source cookie. Within the store, however, such variants eventually end up being converted to the HostOnly case per the RFC.

Is their utility in retaining this distinction? As it stands, there is logic around this in CookieStore::insert() that basically assumes this is an invalid case (resulting in CookieError::UnspecifiedDomain). It may make sense to simplify by removing this particular variant.