code: function PDFOutlineViewer_addToggleButton(div) {
              var toggler = document.createElement('div');
              toggler.className = 'outlineItemToggler';
              toggler.onclick = function (event) {
                event.stopPropagation();
                toggler.classList.toggle('outlineItemsHidden');
                if (event.shiftKey) {
                  var shouldShowAll = !toggler.classList.contains('outlineItemsHidden');
                  this._toggleOutlineItem(div, shouldShowAll);
                }
              }.bind(this);
              div.insertBefore(toggler, div.firstChild);
            }
docs: Prepend a button before an outline item which allows the user to toggle
  the visibility of all outline items at that level.
  
  @private