controlBackgroundAttribute_(b1, b2) {
    let backgroundColor = shaka.cea.CeaUtils.DEFAULT_BG_COLOR;
    if ((b1 & 0x07) === 0x0) {
      // If background provided, last 3 bits of b1 are |0|0|0|. Color is in b2.
      backgroundColor = shaka.cea.Cea608DataChannel.BG_COLORS[(b2 & 0xe) >> 1];
    }
    this.curBuf_.setBackgroundColor(backgroundColor);
  }
Background attribute control code handler.
@param {number} b1 Byte #1
@param {number} b2 Byte #2.
@private