Opaque origins are meant to be unique in terms of equality from
one another. Since this uniqueness needs to be across processes,
use a nonce to implement the uniqueness check.
Instead, porting over all users to use the newly created
Origin::create_opaque factory function. This also requires porting
over some users of Origin to avoid default construction.
The spec seems to indicate in its wording that while opaque
origins only serialize to 'null', they can still be tested
for equality with one another. Probably we will need to
generate some unique ID which is unique across processes.
