getDefaultScrollBorders = function() {
    var containerDimensions = $(this).data(pluginName).containerDimensions;
    return [
        containerDimensions.top,
        containerDimensions.left + containerDimensions.width,
        containerDimensions.top + containerDimensions.height,
        containerDimensions.left];
  }
Returns the default top, right, bottom and left positions of the viewport
borders that should trigger scrolling when the selection box is dragged
close to them, if the scroller does not specify any in its
'getScrollBorders' method.

@return {!Array.<number>} Positions of the scrolling viewport borders,
    respectively in the following order: top, right, bottom, left. All
    positions are relative to the top-left corner of the document. The
    default are the border positions of the selection container itself.