[sensors] SensorReadingEventInit's reading member needs to be required

foolip has just created a new issue for 
https://github.com/w3c/sensors:

== SensorReadingEventInit's reading member needs to be required ==
https://w3c.github.io/sensors/#the-sensor-reading-event-interface

In other words:
```WebIDL
dictionary SensorReadingEventInit : EventInit {
  required SensorReading reading;
};
```

Otherwise, `new SensorReadingEvent('type', {})` will not throw a 
TypeError, and the resulting instance's `reading` attribute would have
 to return null, but it's not nullable (and need not be).

Please view or discuss this issue at 
https://github.com/w3c/sensors/issues/129 using your GitHub account