private void OnInAppRequest(string productId)
 {
     Debug.Log("In App Request for product Id: " + productId + " received");
     //Then you should perfrom purchase  for this product id, using this or another game billing plugin
     //Once the purchase is complete, you should call RecordInAppResolution with one of the constants defined in GADInAppResolution:
     GoogleMobileAd.RecordInAppResolution(GADInAppResolution.RESOLUTION_SUCCESS);
 }