I talked about this with bz. The issue is that we have a lot of XUL reftests
that end up getting run as remote XUL given that the reftest harness loads
them with file:// URIs. But realistically most of them probably want to test
the fully-featured XBL environment that we provide to frontend and extensions.
So the compromise here is to do XBL scopes for content, and no XBL scopes
for reftests/crashtests.
