static getTextContents(node) {
    const StringUtils = shaka.util.StringUtils;
    if (typeof node === 'string') {
      return StringUtils.htmlUnescape(node);
    }
    const textContent = node.children.reduce(
        (acc, curr) => (typeof curr === 'string' ? acc + curr : acc),
        '',
    );
    if (textContent === '') {
      return null;
    }
    return StringUtils.htmlUnescape(textContent);
  }
Gets inner text.
@param {!shaka.extern.xml.Node | string} node The XML element.
@return {?string} The text contents, or null if there are none.