createMediaSourceEngine(mediaElement, textDisplayer, playerInterface,
      lcevcDec, config) {
    return new shaka.media.MediaSourceEngine(
        mediaElement,
        textDisplayer,
        playerInterface,
        config,
        lcevcDec);
  }
Create a new media source engine. This will ONLY be replaced by tests as a
way to inject fake media source engine instances.

@param {!HTMLMediaElement} mediaElement
@param {!shaka.extern.TextDisplayer} textDisplayer
@param {!shaka.media.MediaSourceEngine.PlayerInterface} playerInterface
@param {shaka.lcevc.Dec} lcevcDec
@param {shaka.extern.MediaSourceConfiguration} config

@return {!shaka.media.MediaSourceEngine}