static findChild(elem, name) {
    const children = shaka.util.TXml.findChildren(elem, name);
    if (children.length != 1) {
      return null;
    }
    return children[0];
  }
Finds a child XML element.
@param {!shaka.extern.xml.Node} elem The parent XML element.
@param {string} name The child XML element's tag name.
@return {shaka.extern.xml.Node | null} The child XML element,
  or null if a child XML element
  does not exist with the given tag name OR if there exists more than one
  child XML element with the given tag name.