We have a very nice and robust infrastructure for per-origin expando sharing
for Xrays. Unfortunately, the only way to currently exercise it is with
Location objects, since those are the only objects with same-origin Xrays
(cross-origin Xrays don't allow expandos at all). Sandbox wantXrays would
almost work here, but we actually make an explicit exception for sandboxes
so that they never share expandos (the 'exlusive global' stuff).
I think the infrastructure is nice and that we may want it in the future, so
I don't really want to back it out. But I also don't want to leave it in the
tree untested. So I'm adding an explicit Cu API to force DOM compartments to
use same-origin Xrays. This allows us to keep testing this stuff, which I think
is important.
