addA11yLoadEvent gets the contentWindow and wait for the document from that window to fire the load event.
Enabling the DocumentChannel for parent process load (or here in non-e10s mode) we have one extra event loop before the load starts.
So the window passed to addAllyLoadEvent would have been of the about:blank page.
The current code was based on an observable behaviour which was that the load was occuring synchronously.
DocumentChannel broke that assumption.
Differential Revision: https://phabricator.services.mozilla.com/D70799
8ed0970932