function getDocumentScrollElement(doc) {
  doc = doc || document;
  if (doc.scrollingElement) {
    return doc.scrollingElement;
  }
  return !isWebkit && doc.compatMode === 'CSS1Compat' ?
    doc.documentElement :
    doc.body;
}
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}