code: function isOverlay_(obj) {
    var o = obj;
    if (isArray_(obj) && obj.length > 0) {
      o = obj[0];
    }
    if (isArray_(o) && o.length > 0) {
      o = o[0];
    }
    if (o instanceof G.LatLng || o instanceof G.Marker ||
    o instanceof G.Polyline ||
    o instanceof G.Polygon ||
    o instanceof G.LatLngBounds) {
      return true;
    }
    return false;
  }
docs: Is the object an Google Overlay?
  @param {Object} obj
  @return {Boolean}