Mark click_iframe_crossorigin.sub.html as flaky on Mac

Despite efforts to address it, this test continues to be flaky on Mac,
so squash the flaky-failures.

Bug: 1137228
Change-Id: I48e4745d4f21977b59cfeba99e73d39126932ad7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502312
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821269}

third_party/blink/web_tests/TestExpectations

@@ -6285,6 +6285,8 @@ crbug.com/1138591 [ Mac10.15 ] http/tests/dom/raf-throttling-out-of-view-cross-o
 # WebRTC: Payload demuxing times out in Plan B. This is expected.
 crbug.com/1139052 virtual/webrtc-wpt-plan-b/external/wpt/webrtc/protocol/unbundled-pt-demuxing.https.html [ Timeout ]
 
+crbug.com/1137228 [ Mac ] external/wpt/infrastructure/testdriver/click_iframe_crossorigin.sub.html [ Pass Failure ]
+
 # Rename document.featurePolicy to document.permissionsPolicy
 crbug.com/1123116 external/wpt/permissions-policy/payment-supported-by-permissions-policy.tentative.html [ Failure ]
 crbug.com/1123116 external/wpt/permissions-policy/permissions-policy-frame-policy-allowed-for-all.https.sub.html [ Failure ]