static getElementsByTagName(elem, name, found = []) {
    if (elem.tagName === name) {
      found.push(elem);
    }
    if (elem.children) {
      for (const child of elem.children) {
        shaka.util.TXml.getElementsByTagName(child, name, found);
      }
    }
    return found;
  }
Finds child XML elements recursively.
@param {!shaka.extern.xml.Node} elem The parent XML element.
@param {string} name The child XML element's tag name.
@param {!Array<!shaka.extern.xml.Node>} found accumulator for found nodes
@return {!Array<!shaka.extern.xml.Node>} The child XML elements.