function getScrollOffset() {
      return {top: 0, left: 0};
    }
Returns the offset, in pixels, that should be added to selectable
elements' positions (as computed by the plugin 'positioner'), to take
into account scrolling. This is not relevant when native browser
scrolling is used.

@return {!Object.<string, number>} An object containing the 'top' and
    'left' properties, pointing respectively to the top and left offset
    to add.