Bug 315056 – [Site Isolation] http/tests/security/cross-origin-blob-transfer.html is flaky on post-commit CI bots

• Component: New Bugs
• Status: RESOLVED FIXED

[Anthony Tarbinian]

http/tests/security/cross-origin-blob-transfer.html is flaky on macOS site isolation bots with the following diff:

```
--- /Volumes/Data/worker/Apple-Tahoe-Release-WK2-Site-Isolation-Tree-Tests/build/layout-test-results/http/tests/security/cross-origin-blob-transfer-expected.txt
+++ /Volumes/Data/worker/Apple-Tahoe-Release-WK2-Site-Isolation-Tree-Tests/build/layout-test-results/http/tests/security/cross-origin-blob-transfer-actual.txt
@@ -2,16 +2,4 @@
 
 TEST COMPLETE
 
-Summary
-
-Harness status: OK
-
-Found 1 tests
-
-1 Pass
-Details
-
-Result	Test Name	Message
-Pass	Test for creating blob in iframe and then transferring it cross-origin.	
-Asserts run
-
+Running, 1 complete, 0 remain
```

This failure happens since the test uses both testharness and WKTR to indicate when the test is complete:
```
  window.addEventListener("message", (e) => {
      e.data.text().then(t => {
          assert_equals(t, "Blob content")
          test.done();                      // (A) testharness completion
          if (self.testRunner)
              testRunner.notifyDone();      // (B) WKTR completion
      });
  });
```

This leads to a potential race where the DOM gets dumped (B) before testharness's timer 
(A) gets a chance to render the summary table. The dump captures only the in-progress
 Running, 1 complete, 0 remain status line that testharness writes during the test.

[Radar WebKit Bug Importer]

<rdar://problem/177382438>

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/65143

[EWS]

Committed 313508@main (fff9c1acbb11): <https://commits.webkit.org/313508@main>

Reviewed commits have been landed. Closing PR #65143 and removing active labels.