Bug 1842030 - Part 2: Check the global private browsing state for...

Bug 1842030 - Part 2: Check the global private browsing state for HttpBaseChannel::IsBrowsingContextDiscarded() if the loadGroup is not avaiable. r=necko-reviewers,jesup

The HttpBaseChannel::IsBrowsingContextDiscarded() did always return
false if the loadGroup is not avaiable. But, this may not be correct for
the private channels if the private session has been ended. To fix this,
we make the function to check the global private browsing state
if the loadGroup is not available for private channels.

Depends on D184010

Differential Revision: https://phabricator.services.mozilla.com/D184479