static logPrefix_(mediaState) {
    return '(' + mediaState.type + ':' + mediaState.stream.id + ')';
  }
@param {shaka.media.StreamingEngine.MediaState_} mediaState
@return {string} A log prefix of the form ($CONTENT_TYPE:$STREAM_ID), e.g.,
  "(audio:5)" or "(video:hd)".
@private