Update simplified_geofence.txt to what I understand is what we agreed on.

The ongeofenceerror event will probably need to be worked out more, but (after you left) we all seemed to agree that having such an event will be necessary. Also I now noticed you moved the GeofencingController to be exposed on the ServiceWorkerGlobalScope? I think it is much nicer to keep this on ServiceWorkerRegistration, to allow registering/unregistering of fences from web pages directly.