[Sheriff] Marking tests as flaky for Mac

TBR=elklm@google.com

https://crrev.com/c/2421731 marked
external/wpt/xhr/xhr-timeout-longtask.any.html as flaky on Mac10.14, but
the same flakiness is present on Mac10.13:

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/20098

Bug: 1130533
Change-Id: I801951c158677be84804764f41bbf9b8fe6de6bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422924
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Chris Hall <chrishall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809158}

third_party/blink/web_tests/TestExpectations

@@ -6736,4 +6736,6 @@ virtual/scroll-unification/fast/scrolling/scrollbars/dsf-ready/mouse-interaction
 
 # Sheriff 2020-09-21
 crbug.com/1130500 [ Mac10.13 Debug ] virtual/omt-worker-fetch/external/wpt/xhr/xhr-timeout-longtask.any.worker.html [ Pass Failure ]
-crbug.com/1130533 [ Mac10.14 ] external/wpt/xhr/xhr-timeout-longtask.any.html [ Pass Failure ]
+
+# Sheriff 2020-09-22
+crbug.com/1130533 [ Mac ] external/wpt/xhr/xhr-timeout-longtask.any.html [ Pass Failure ]