Marking test as flaky for Mac10.13 Debug.

Bug: 1130500
Change-Id: I9354e349132dfda8359d761ff84ed10016d7914b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421492Reviewed-by: Illia Klimov <elklm@google.com>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Illia Klimov <elklm@google.com>
Cr-Commit-Position: refs/heads/master@{#808828}

third_party/blink/web_tests/TestExpectations

@@ -6747,3 +6747,6 @@ virtual/scroll-unification/fast/scrolling/scrollbars/mouse-scrolling-on-div-scro
 virtual/scroll-unification/fast/scrolling/scrollbars/scrollbar-occluded-by-div.html [ Failure ]
 virtual/scroll-unification/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.html [ Failure ]
 virtual/scroll-unification/fast/scrolling/scrollbars/dsf-ready/mouse-interactions-dsf-2.html [ Failure ]
+
+# 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 ]
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