function _getWinSize() {
    if (window.innerWidth != undefined) {
      return { width: window.innerWidth, height: window.innerHeight };
    } else {
      var D = document.documentElement;
      return { width: D.clientWidth, height: D.clientHeight };
    }
  }
Provides a cross-browser way to get the screen dimensions
via: http://stackoverflow.com/questions/5864467/internet-explorer-innerheight

@api private
@method _getWinSize
@returns {Object} width and height attributes