Add track_list member to AudioTrack, VideoTrack, TextTrack structs

Add member to the track structs pointing at their associated tracklist
and update it when the track is added or removed from a tracklist.

components/script/dom/htmltrackelement.rs

@@ -59,6 +59,7 @@ impl HTMLTrackElement {
             Default::default(),
             Default::default(),
             Default::default(),
+            None,
         );
         Node::reflect_node(
             Box::new(HTMLTrackElement::new_inherited(