setContentArtist(artist) {
    if (!this.metadata_) {
      this.metadata_ = {};
    }
    this.metadata_['artist'] = artist;
    this.metadata_['metadataType'] =
        cast.receiver.media.MetadataType.MUSIC_TRACK;
  }
Set the Cast content's artist.
Also sets the metadata type to music.
Should be called from an appDataCallback.

@param {string} artist
@export