Bug 1489385 part 1 - Have MozDOMFullscreen::NewOrigin event de-aligned with refresh driver. r=smaug

In following patches, we are changing how fullscreen events dispatches,
and this patch removes the only dependency of the original mechanism
outside content-exposed fullscreen events.

Listeners of NewOrigin event nowadays just send a message to the chrome,
so it doesn't really matter a lot when is it dispatched.

This event was made that way as part of bug 1168705. I cannot recall
the exact reason for doing so, but maybe that time we still care about
non-e10s, and syncing this event with flush can help merging the browser
chrome UI change into the same flush on non-e10s.

Differential Revision: https://phabricator.services.mozilla.com/D5234