Implement support for selecting media element source from children

https://github.com/servo/servo/blob/9e2bbb73858b4a3e83cd1397906eaad789749a9c/components/script/dom/htmlmediaelement.rs#L536

See the "Otherwise (mode is children)" branch of step 9 at https://html.spec.whatwg.org/multipage/media.html#concept-media-load-algorithm.