Function obtain_response 

async fn obtain_response(
    client: &ServoClient,
    url: &ServoUrl,
    method: &Method,
    request_headers: &mut HeaderMap,
    body_sender: Option<Arc<Mutex<Option<IpcSender<BodyChunkRequest>>>>>,
    source_is_null: bool,
    pipeline_id: &Option<PipelineId>,
    request_id: Option<&str>,
    destination: Destination,
    is_xhr: bool,
    context: &FetchContext,
    fetch_terminated: UnboundedSender<bool>,
    browsing_context_id: Option<BrowsingContextId>,
) -> Result<(Response<Decoder>, Option<ChromeToDevtoolsControlMsg>), NetworkError>

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