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/texttracklist.rs

@@ -94,6 +94,7 @@ impl TextTrackList {
                 }),
                 &canceller,
             );
+            track.add_track_list(self);
         }
     }
 
@@ -101,6 +102,9 @@ impl TextTrackList {
     // removed from the TextTrackList.
     #[allow(dead_code)]
     pub fn remove(&self, idx: usize) {
+        if let Some(track) = self.dom_tracks.borrow().get(idx) {
+            track.remove_track_list();
+        }
         self.dom_tracks.borrow_mut().remove(idx);
         self.upcast::<EventTarget>()
             .fire_event(atom!("removetrack"));