When using the Google Maps API for android or ios it seems that the indoor maps are enabled by default and the \"floor picker\" control automatically appears when you zoom in to
Google doesn't support this in the Javascript API currently.
These three issues all have responses from Google stating that it's an "Enhancement", which may mean they'll add it sometime:
https://code.google.com/p/gmaps-api-issues/issues/detail?id=6642
https://code.google.com/p/gmaps-api-issues/issues/detail?id=6792
https://code.google.com/p/gmaps-api-issues/issues/detail?id=6783
The only workaround I'm aware of is to manually add your own floor picker, then add each floor as an ImageMapType and switch between them. Unfortunately, this requires you to provide all of the floor imagery yourself rather than use Google's.
ImageMapType docs: https://developers.google.com/maps/documentation/javascript/examples/maptype-image-overlay