Revert "[Credentialless] Fix flakes about iframeTest.js"

This reverts commit cad80cd16f74475116e60b568f0fce785fb50f99.

Reason for revert: crbug.com/1217587

Original change's description:
> [Credentialless] Fix flakes about iframeTest.js
>
> In the various tests, there are up to 8 <iframe> navigating concurrently.
>
> There are no ways to detect an <iframe> fails to load other than using
> a timeout. See whatwg/html#125.
>
> The 3000ms timeout was sometimes too short. This cause flakes.
> To reduce the rate of false positive, the timeout has been increased to
> 6000ms.
>
> R: [email protected]
> CC: [email protected]
> Fixed: 1215956
> Bug: 1175099
> Change-Id: I7bedccb351ff62d55b6020403aac5457608e5497
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2944929
> Reviewed-by: Titouan Rigoudy <[email protected]>
> Commit-Queue: Arthur Sonzogni <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#890171}

Bug: 1175099
Change-Id: I258c8e9219c708f22e6d14d2648e6e588a410b99
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2944393
Owners-Override: Saurabh Nijhara <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Commit-Queue: Saurabh Nijhara <[email protected]>
Cr-Commit-Position: refs/heads/master@{#890202}
NOKEYCHECK=True
GitOrigin-RevId: cd1ffb10d85a122bfbe7747a4411eaff45f9c751

blink/web_tests/TestExpectations

@@ -3514,6 +3514,9 @@ crbug.com/626703 external/wpt/css/vendor-imports/mozilla/mozilla-central-reftest
 crbug.com/626703 external/wpt/encoding/eof-utf-8-three.html [ Failure ]
 crbug.com/626703 external/wpt/encoding/eof-utf-8-two.html [ Failure ]
 crbug.com/626703 external/wpt/html/browsers/windows/noreferrer-window-name.html [ Timeout ]
+crbug.com/1215956 [ Linux ] external/wpt/html/cross-origin-embedder-policy/credentialless/iframe-coep-credentialless.tentative.https.html [ Pass Failure ]
+crbug.com/1215956 [ Linux ] external/wpt/html/cross-origin-embedder-policy/credentialless/iframe-coep-require-corp.tentative.https.html [ Pass Failure ]
+crbug.com/1215956 [ Linux ] external/wpt/html/cross-origin-embedder-policy/credentialless/iframe-coep-none.tentative.https.html [ Pass Failure ]
 crbug.com/626703 external/wpt/html/semantics/embedded-content/the-embed-element/embed-represent-nothing-04.html [ Failure ]
 crbug.com/626703 external/wpt/html/semantics/embedded-content/the-video-element/video_initially_paused.html [ Failure ]
 crbug.com/626703 external/wpt/html/semantics/grouping-content/the-li-element/grouping-li-reftest-list-owner-menu.html [ Failure ]

blink/web_tests/external/wpt/html/cross-origin-embedder-policy/credentialless/resources/iframeTest.js

@@ -61,7 +61,7 @@ const iframeTest = function(
     // There are no interoperable ways to check an iframe failed to load. So a
     // timeout is being used.
     // See https://github.com/whatwg/html/issues/125
-    step_timeout(()=>send(test_token, "block"), 6000);
+    step_timeout(()=>send(test_token, "block"), 3000);
 
     assert_equals(await receive(test_token), expectation);
   }, description);