docs: function IFrameMessageSender(iframe) {
    if (!iframe) {
      console.error('No iframe specified');
      return;
    }
    this.iframe = iframe;
  
    // On iOS, if the iframe is across domains, also send DeviceMotion data.
    if (this.isIOS_()) {
      window.addEventListener('devicemotion', this.onDeviceMotion_.bind(this), false);
    }
  }