parseMpdChaining_(mpd) {
    const TXml = shaka.util.TXml;
    const supplementalProperties =
        TXml.findChildren(mpd, 'SupplementalProperty');

    if (!supplementalProperties.length) {
      return null;
    }

    for (const prop of supplementalProperties) {
      const schemeId = prop.attributes['schemeIdUri'];
      if (schemeId == 'urn:mpeg:dash:chaining:2016') {
        return prop.attributes['value'];
      }
    }

    return null;
  }
Reads chaining url.

@param {!shaka.extern.xml.Node} mpd
@return {?string}
@private