This reverts commit 4a03c6c459fdbf11976a424aa02a1d094484134c.

Reason for revert:

This has caused tests in upstream WPT to fail, blocking unrelated PRs. It was still possible to upstream this because those tests weren't triggered on the change due to a bug:
#37623

There was an attempted fix for this:
#37549

But, quoting jgraham from the WPT Matrix chat:

> the actual fix failed a test I wrote and now I need to spend some more time investigating

Original change's description:
> WPT: Allow `window.onload` to contain multiple `test()`s
>
> ******************************************************************
> *** SHERIFFS: please don't revert this CL if it causes web_tests
>               to flake/fail. If that happens, the cause is a bad
>               test. Please mark that test as flaky/fail in
>               TestExpectations, with a new crbug. Please block the
>               new bug against crbug.com/1395228. Thanks!
> ******************************************************************
>
> Prior to this CL, a test like this:
>
> ```
> <script>
> window.onload = () => {
>   test((t) => { ... }, 'test 1');
>   test((t) => { ... }, 'test 2');
>   test((t) => { ... }, 'test 3');
> };
> </script>
> ```
>
> would not run anything after test #1. The issue is that the testharness
> immediately adds a window load handler that marks `all_loaded = true`,
> and that ends the tests as soon as the first result from the first test
> is processed. (The test runner waits for the first test because
> `Tests.prototype.all_done()` also waits until `this.tests.length > 0`.)
> There were various mitigating corner cases, such as if you started
> the list of tests with a promise_test(), that would increment a
> counter that kept the rest of the tests alive. Etc.
>
> With this CL, the testharness-added window.onload handler runs a
> setTimeout(0), so that `all_loaded` is only set to true after all of
> the tests are loaded by any window.onload handler.
>
> This exposed a few tests that should have been failing but were
> masked by the lack of test coverage - bugs have been filed for
> those. Also, several tests that were working around this via various
> means are also cleaned up in this CL. I'm sure there are more of
> those.
>
> Bug: 1395228,1395226,1307772
> Change-Id: I6f12b5922186af4e1e06808ad23b47ceac68559c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4074305
> Reviewed-by: Weizhong Xia <weizhong@google.com>
> Auto-Submit: Mason Freed <masonf@chromium.org>
> Reviewed-by: Mason Freed <masonf@chromium.org>
> Commit-Queue: Mason Freed <masonf@chromium.org>
> Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1081558}

Bug: 1395228,1395226,1307772
Change-Id: Icbddad3a8bb47473bcbc331f424661b9041addf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4111318
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1085925}