checkDisplays() {
    return WebVRManager.getVRDisplay()
      .then((display) => {
        this.defaultDisplay = display;
        this.__setState(State.READY_TO_PRESENT);
        return display;
      })
      .catch((e) => {
        delete this.defaultDisplay;
        if (e.name == 'NO_DISPLAYS') {
          this.__setState(State.ERROR_NO_PRESENTABLE_DISPLAYS);
        } else if (e.name == 'WEBVR_UNSUPPORTED') {
          this.__setState(State.ERROR_BROWSER_NOT_SUPPORTED);
        } else {
          this.__setState(State.ERROR_UNKOWN);
        }
      });
  }
Check if the browser is compatible with WebVR and has headsets.
@return {Promise<VRDisplay>}