Add support for converting Geolocation Position+Coordinates to JSON (object)

[marcoscaceres]

Given GeolocationCoordinates are just nullable doubles, it would be nice if GeolocationCoordinates exposed a (WebIDL) default .toJSON() method.

[reillyeon]

That would be nice given that JSON.stringify() does not work on GeolocationCoordinates.

[marcoscaceres]

Another question is if we should also add a toJSON() method on GeolocationPosition? I don't know if there is much value to getting the timestamp...

[reillyeon]

Less value but it would still feel weird if you could stringify the coordinates but not the full object passed to your callback so I say yes, let's add a toJSON() method to both interfaces.

[marcoscaceres]

Sounds good.