code: function getDocumentScrollElement(doc) {
    doc = doc || document;
    if (doc.scrollingElement) {
      return doc.scrollingElement;
    }
    return !isWebkit && doc.compatMode === 'CSS1Compat' ?
      doc.documentElement :
      doc.body;
  }
docs: Gets the element with the document scroll properties such as `scrollLeft` and
  `scrollHeight`. This may differ across different browsers.
  
  NOTE: The return value can be null if the DOM is not yet ready.
  
  @param {?DOMDocument} doc Defaults to current document.
  @return {?DOMElement}