createDrmEngine(playerInterface) {
    return new shaka.drm.DrmEngine(playerInterface);
  }
Create a new DrmEngine instance. This may be replaced by tests to create
fake instances. Configuration and initialization will be handled after
|createDrmEngine|.

@param {shaka.drm.DrmEngine.PlayerInterface} playerInterface
@return {!shaka.drm.DrmEngine}