static getAttributeNSList(elem, nsList, name) {
    for (const ns of nsList) {
      const attr = shaka.util.TXml.getAttributeNS(
          elem, ns, name,
      );
      if (attr) {
        return attr;
      }
    }
    return null;
  }
Gets a namespace-qualified attribute.
@param {!shaka.extern.xml.Node} elem The element to get from.
@param {!Array<string>} nsList The lis of namespace URIs.
@param {string} name The local name of the attribute.
@return {?string} The attribute's value, or null if not present.