requestExit() {
    const initialState = this.state;

    return new Promise((resolve, reject)=> {
      if (this.options.onRequestStateChange(State.READY_TO_PRESENT)) {
        return this.options.beforeExit()
          .then(()=>
            // if we were presenting VR, exit VR, if we are
            // exiting fullscreen, exit fullscreen
            initialState === State.PRESENTING ?
              this.manager.exitVR(this.manager.defaultDisplay) :
              this.manager.exitFullscreen())
          .then(resolve);
      } else {
        reject(new Error(State.ERROR_REQUEST_STATE_CHANGE_REJECTED));
      }
    });
  }
Request exiting presentation mode
@return {Promise}