static getChildNodes(elem) {
    const found = [];
    if (!elem.children) {
      return [];
    }
    for (const child of elem.children) {
      if (typeof child !== 'string') {
        found.push(child);
      }
    }
    return found;
  }
gets child XML elements.
@param {!shaka.extern.xml.Node} elem The parent XML element.
@return {!Array<!shaka.extern.xml.Node>} The child XML elements.