Bug 1816158 - Part 1: Disallow GC while iterating global's debugger vector r=sfink

GC can mutate this vector so don't allow that while we are iterating. I think
it would be safe to use index-based iteration but it's safer to just ban it
entirely.

This fixes the crash produced by the testcase.

Differential Revision: https://phabricator.services.mozilla.com/D169701