Currently Google Maps JavaScript API doesn't expose any boundaries of geographic features. There is very old feature request in the public issue tracker to add this functionality, however it looks like Google didn't set high priority on this task:
https://issuetracker.google.com/issues/35816953
Feel free to star the public feature request to express your interest and subscribe to notifications.
Also, you can find a workaround to get polygons from other sources and add them to Google maps as additional layers.
The nice workaround to get polygons in GeoJSON format from OSM is described in the following answer:
https://stackoverflow.com/a/40172098/5140781
So, if you download the GeoJSON you will be able to add it to map using the data layer and its loadGeoJson() method:
https://developers.google.com/maps/documentation/javascript/datalayer#load_geojson
I hope this helps!
