This changes how ProcessIsolation handles the generic "web" remote type
within private browsing windows or container tabs, such that these
processes are now allocated as `web=^userContextId=1` or similar, rather
than the generic `web` process.
This shouldn't change process isolation for most content, but will impact
how things like toplevel data URIs are loaded within private browsing
windows or container tabs.
Differential Revision: https://phabricator.services.mozilla.com/D282085
Nika Layzell
a77c97abcb