The XPInstall content handlers currently make all sorts of assumptions about
the process they're being run in and the docShell their request is tied to.
These assumptions don't hold under Fission, either now, when they may be
called just after a process switch, or in the future, when they'll be called
in the parent process.
This patch updates them to operate on the BrowsingContext of the request, and
to do their messaging via the process message manager, rather than via frame
message managers. As a bonus, it also removes the existing hacks they had in
place to handle the cases where frame message managers didn't work.
Differential Revision: https://phabricator.services.mozilla.com/D48224
24898be019