Treat the Cardboard Button as a VR controller
Treat the Cardboard Button as a VR controller

This device does not have a pose, but it does have a displayId,
so add it to vrGamepads and add some sanity checks so that other
code that was expecting a pose doesn't break.

toji/webvr.info,paezagon/webvr-samples,toji/webvr.info,paezagon/webvr-samples