[Credentialless]: Make fetch.tentative.https.html more reliable.

Previously, the test was assuming the window can be closed immediately
after sending the last fetch request.
This is not true, because closing the window might interrupt previously
scheduled fetch requests and fails the test.
We need to wait for the test completion before assuming we can close the
windows.

I did a local test with 100 wpt run:
- without fix: 16/100 failures.
- with fix: 0/100 failures.

Bug: 1175099,1198255
Change-Id: I70a56ba4821d46d12f45c38d8850c876c5957649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2825904
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872837}

html/cross-origin-embedder-policy/credentialless/cors-or-credentialless/fetch.tentative.https.html

@@ -23,12 +23,14 @@
   const w_control_url = same_origin + executor_path +
     coep_none + `&uuid=${w_control_token}`
   const w_control = window.open(w_control_url);
+  add_completion_callback(() => w_control.close());
 
   // One window with COEP:credentialless. (experiment)
   const w_credentialless_token = token();
   const w_credentialless_url = same_origin + executor_path +
     coep_credentialless + `&uuid=${w_credentialless_token}`;
   const w_credentialless = window.open(w_credentialless_url);
+  add_completion_callback(() => w_credentialless.close());
 
   const fetchTest = function(
     description, origin, mode, credentials,
@@ -123,10 +125,6 @@
     cross_origin, 'no-cors', 'same-origin',
     undefined,
     undefined);
-
-  // Cleanup. Safe, because scheduled after every requests from `fetchTest`.
-  send(w_control_token, `close()`);
-  send(w_credentialless_token, `close()`);
 }, "");
 
 </script>