function makeVariant(id, audio, video) {
    return {
      allowedByApplication: true,
      allowedByKeySystem: true,
      audio: audio,
      bandwidth: 1024,
      id: id,
      disabledUntilTime: 0,
      language: '',
      primary: false,
      video: video,
      decodingInfos: [],
    };
  }
Create a variant where the audio stream is optional but the video stream
is required. For the cases where audio and video are in the same stream,
it should be provided as the video stream.

@param {number} id
@param {?shaka.extern.Stream} audio
@param {shaka.extern.Stream} video
@return {shaka.extern.Variant}