Currently MSE stream parsers create MediaTrack objects before MediaSourceState creates DemuxerStreams, which makes it necessary to maintain a map in MediaTracks collection to eventually allow translating blink track ids into DemuxerStream objects (see MediaTrack::SetDemuxerStreamForMediaTrack and ::OnTrackIdsAssigned). Perhaps we can simplify this by passing an std::vector of MediaTrack objects from stream parsers into MediaSourceState::OnNewConfigs and then set up all the necessary mappings there.