generateRegionId_(region) {
    const percentageUnit = shaka.text.CueRegion.units.PERCENTAGE;
    const heightUnit = region.heightUnits == percentageUnit ? '%' : 'px';
    const viewportAnchorUnit =
        region.viewportAnchorUnits == percentageUnit ? '%' : 'px';
    const uniqueRegionId = `${region.id}_${
      region.width}x${region.height}${heightUnit}-${
      region.viewportAnchorX}x${region.viewportAnchorY}${viewportAnchorUnit}`;

    return uniqueRegionId;
  }
Compute a unique internal id:
Regions can reuse the id but have different dimensions, we need to
consider those differences
@param {shaka.text.CueRegion} region
@private