Bug 1866876 [wpt PR 43376] - Add infra test that window.load happens …

…before reftest capture, a=testonly

Automatic update from web-platform-tests
Add infra test that window.load happens before reftest capture

This adds a WPT infrastructure test to enforce that the harness
waits for window.load before capturing reftest images. That allows
testing like this:

```
<link rel=match href=green.html>
<script>
  window.onload = () => document.body.append('test');
</script>
```

See [1] for conversation.

[1] web-platform-tests/wpt#43310

Change-Id: I1f3f9836ca880350b294538dd2f2d58689485f7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5064229
Reviewed-by: Weizhong Xia <weizhonggoogle.com>
Commit-Queue: Mason Freed <masonfchromium.org>
Cr-Commit-Position: refs/heads/main{#1229618}

--

wpt-commits: aff9e54e2b07b57885c89e8d2175570946a9e5bc
wpt-pr: 43376

UltraBlame original commit: 12454e2d818196a790f2711897bc1424a98ecb86

testing/web-platform/tests/infrastructure/reftest/reftest_window_load.html

@@ -0,0 +1,85 @@
+<
+!
+DOCTYPE
+html
+>
+<
+title
+>
+rel
+=
+match
+waits
+for
+window
+load
+<
+/
+title
+>
+<
+link
+rel
+=
+match
+href
+=
+green
+.
+html
+>
+<
+script
+>
+window
+.
+onload
+=
+(
+)
+=
+>
+{
+const
+s
+=
+document
+.
+createElement
+(
+'
+style
+'
+)
+;
+s
+.
+innerText
+=
+'
+:
+root
+{
+background
+-
+color
+:
+green
+}
+'
+;
+document
+.
+body
+.
+appendChild
+(
+s
+)
+;
+}
+;
+<
+/
+script
+>