Mark the WPT flaky.

third_party/blink/web_tests/external/wpt/workers/Worker_script_mimetype.htm
sometimes causes timeout.

Bug: 1422305
Change-Id: I405aa45f65a1fb3103548accd5d672e448ec1ebc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4337184
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1117312}

third_party/blink/web_tests/TestExpectations

@@ -6889,3 +6889,6 @@ crbug.com/1422630 [ Fuchsia ] external/wpt/long-animation-frame/tentative/loaf-s
 crbug.com/1422630 [ Mac ] external/wpt/long-animation-frame/tentative/loaf-script-block.html [ Failure Pass Timeout ]
 crbug.com/1422630 [ Win ] external/wpt/long-animation-frame/tentative/loaf-script-block.html [ Failure Pass ]
 crbug.com/1422630 external/wpt/long-animation-frame/tentative/loaf-desired-exec-time.html [ Failure Pass ]
+
+# Flaky timeout on mac.
+crbug.com/1422305 [ Mac ] external/wpt/workers/Worker_script_mimetype.htm [ Pass Timeout ]