Allow to propagate loadgroup flags to parent process.
As per discussion. Test is in the other patch, assuming this is green
I'll incorporate it, but gotta go for the day.
Differential Revision: https://phabricator.services.mozilla.com/D224824
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1921972
gecko-commit: 611e44883688271ee3aaba71493b9c6ac1cf9f7d
gecko-reviewers: nika, necko-reviewers, valentin