Function obtain_response 

async fn obtain_response(
    fetch_params: &mut FetchParams,
    client: &ServoClient,
    url: &ServoUrl,
    body_sender: Option<Arc<Mutex<Option<IpcSender<BodyChunkRequest>>>>>,
    request_id: Option<&Uuid>,
    context: &FetchContext,
    fetch_terminated: UnboundedSender<bool>,
) -> Result<(Response<Decoder>, Option<ChromeToDevtoolsControlMsg>), NetworkError>

This sets up the callback infrastructure to send body frames to body_sender and fires the client request.