Marking test as flaky for Mac10.14.

TBR=timvolodine@chromium.org

Bug: 1130533
Change-Id: I353b191f652c1ea39a53300226c6b39526f58686
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421731Reviewed-by: Illia Klimov <elklm@google.com>
Commit-Queue: Illia Klimov <elklm@google.com>
Commit-Queue: Illia Klimov <elklm@chromium.org>
Auto-Submit: Illia Klimov <elklm@google.com>
Cr-Commit-Position: refs/heads/master@{#808844}

third_party/blink/web_tests/TestExpectations

@@ -6749,4 +6749,5 @@ virtual/scroll-unification/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.
 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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+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 ]
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