ensure clusterer redraws after zoom

This is supposed to be
```
            this.listeners_ = [
                google.maps.event.addListener(this.getMap(), 'zoom_changed', function () {
                    cMarkerClusterer.resetViewport_(false);
                    // Workaround for this Google bug: when map is at level 0 and '-' of
                    // zoom slider is clicked, a 'zoom_changed' event is fired even though
                    // the map doesn't zoom out any further. In this situation, no 'idle'
                    // event is triggered so the cluster markers that have been removed
                    // do not get redrawn. Same goes for a zoom in at maxZoom.
                    if (this.getZoom() === (this.get('minZoom') || 0) || this.getZoom() === this.get('maxZoom')) {
                        google.maps.event.trigger(this, 'idle');
                    }
                }),
                google.maps.event.addListener(this.getMap(), "idle", function () {
                  cMarkerClusterer.redraw_();
                })
            ];
```