Auto merge of #23342 - Manishearth:webrtc-streams, r=ferjm

Receive streams in WebRTC (and MediaStreamTrack support)

This adds the `ontrack` event handler to webrtc, and all the `MediaStreamTrack` stuff necessary to make it work.

WebRTC has the ability to group media tracks into streams using MSIDs, but I haven't yet figured out how to do this. For now, `ontrack` should work.

This _should_ be complete, but it hasn't yet been tested (hence the WIP)

r? @ferjm or @jdm

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components/script/dom/htmlmediaelement.rs

@@ -839,9 +839,13 @@ impl HTMLMediaElement {
                             self.fetch_request(None);
                         },
                         SrcObject::MediaStream(ref stream) => {
-                            for stream in stream.get_tracks() {
-                                if let Err(_) =
-                                    self.player.borrow().as_ref().unwrap().set_stream(&stream)
+                            for stream in &*stream.get_tracks() {
+                                if let Err(_) = self
+                                    .player
+                                    .borrow()
+                                    .as_ref()
+                                    .unwrap()
+                                    .set_stream(&stream.id())
                                 {
                                     self.queue_dedicated_media_source_failure_steps();
                                 }