Consider adding [SecureContext] for the WebVR interfaces

[kearwood]

Although attention has shifted to WebXR, as the replacement for the WebVR API, some browsers will continue to ship WebVR during this transition period.

I would not want WebVR to act as a gateway to enabling VR experiences on non-secure context, bypassing the WebXR API, which marks all interfaces with [SecureContext].

I would like to propose that the WebVR spec be updated with the addition of [SecureContext] in the WebIDL for all interfaces.