requestEnterFullscreen() {
    return new Promise((resolve, reject)=> {
      if (this.options.onRequestStateChange(State.PRESENTING_FULLSCREEN)) {
        return this.options.beforeEnter()
          .then(()=>this.manager.enterFullscreen(this.sourceCanvas))
          .then(resolve);
      } else {
        reject(new Error(State.ERROR_REQUEST_STATE_CHANGE_REJECTED));
      }
    });
  }
Request entering the site in fullscreen, but not VR
@return {Promise}