Previously, this was passed around using the nsHashPropertyBag base
class on HTTP channels. In bug 1907269, this base was added to
nsExtProtocolChannel to support reading this property for external
protocols.
This unfortunately ended up not working when a HTTP channel redirects to
an external protocol, as the hash property propagation logic in
nsHttpChannel happens after the check that the target channel is a HTTP
channel, so the property was lost when redirecting to an external
protocol.
Rather than refactoring HTTP redirect logic, this patch simplifies by
moving the property onto the LoadInfo where it will be trivially
preserved across redirects, and available in the places which need it.
Because the nsHashPropertyBag base is no longer required for
nsExtProtocolChannel, it is also removed.
Differential Revision: https://phabricator.services.mozilla.com/D220744
793f001a6e