[Autofill Assistant] Add vlog when receiving a network response.

Bug: b/171776026
Change-Id: I061a82a990b60a07952e4c6203b1b6260de97d7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554569
Auto-Submit: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#831016}

components/autofill_assistant/browser/service/service_request_sender_impl.cc

@@ -48,6 +48,8 @@ void OnURLLoaderComplete(
   if (loader->ResponseInfo() && loader->ResponseInfo()->headers) {
     response_code = loader->ResponseInfo()->headers->response_code();
   }
+  VLOG(3) << "Received response: status=" << response_code << ", "
+          << response_str.length() << " bytes";
   std::move(callback).Run(response_code, response_str);
 }