Videos can fire their "canplay" events even when they're not part of the DOM. This
means that videos that have already fired that event won't get the
PictureInPictureToggleChild properly attached after being inserted.
The UAWidgetSetupOrChange event, however, is fired for video events once they've
been bound to the DOM, so we can use that instead.
Differential Revision: https://phabricator.services.mozilla.com/D44541
--HG--
extra : moz-landing-system : lando
