code: function ClusterIcon(cluster, styles, opt_padding) {
    cluster.getMarkerClusterer().extend(ClusterIcon, google.maps.OverlayView);
  
    this.styles_ = styles;
    this.padding_ = opt_padding || 0;
    this.cluster_ = cluster;
    this.center_ = null;
    this.map_ = cluster.getMap();
    this.div_ = null;
    this.sums_ = null;
    this.visible_ = false;
  
    this.setMap(this.map_);
  }
docs: A cluster icon
  
  @param {Cluster} cluster The cluster to be associated with.
  @param {Object} styles An object that has style properties:
  'url': (string) The image url.
  'height': (number) The image height.
  'width': (number) The image width.
  'anchor': (Array) The anchor position of the label text.
  'textColor': (string) The text color.
  'textSize': (number) The text size.
  'backgroundPosition: (string) The background postition x, y.
  @param {number=} opt_padding Optional padding to apply to the cluster icon.
  @constructor
  @extends google.maps.OverlayView
  @ignore