function VRDisplayHMDDevice(display) {
  this.display = display;

  this.hardwareUnitId = display.displayId;
  this.deviceId = 'webvr-polyfill:HMD:' + display.displayId;
  this.deviceName = display.displayName + ' (HMD)';
}
Wraps a VRDisplay and exposes it as a HMDVRDevice