static getLaurl_(xml) {
    const TXml = shaka.util.TXml;
    // LA_URL element is optional and no more than one is
    // allowed inside the DATA element. Only absolute URLs are allowed.
    // If the LA_URL element exists, it must not be empty.
    for (const elem of TXml.getElementsByTagName(xml, 'DATA')) {
      if (elem.children) {
        for (const child of elem.children) {
          if (child.tagName == 'LA_URL') {
            return /** @type {string} */(
              shaka.util.TXml.getTextContents(child));
          }
        }
      }
    }

    // Not found
    return '';
  }
PlayReady Header format: https://goo.gl/dBzxNA

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