This code has been broken in content processes since e10s; i.e. we currently fire doc load complete events for iframes, even if the parent doc is still loading.
Rather than returning to the pre-e10s behaviour, this check has been removed instead for several reasons:
1. Mac wants AXLayoutComplete to be fired on iframes and we map that from doc load complete.
2. We've been doing this since e10s (3+ years) and it doesn't seem to have hurt any screen readers, so presumably, this has been fixed in screen readers.
3. This check can never work for OOP (Fission) iframes, since the parent document is in a different process and we thus can't query its loading state.
4. The behaviour is currently inconsistent between parent process documents and content process documents. Removing this check makes it consistent.
Differential Revision: https://phabricator.services.mozilla.com/D109582
b91f768a81