Implement this method to initialize the overlay DOM elements. This method
is called once after setMap() is called with a valid map. At this point,
panes and projection will have been initialized.
Returns void
onRemove
onRemove():void
Implement this method to remove your elements from the DOM. This method
is called once following a call to setMap(null).
MarkerClusterer creates and manages per-zoom-level clusters for large amounts of markers. See MarkerClustererOptions for more details.