[Sheriff] Mark flaky external/wpt/xhr/xhr-timeout-longtask.any.worker.html

on Mac

First failure on Mac10.13 Tests (dbg)
https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/20097

First failure on Mac10.14
https://ci.chromium.org/p/chromium/builders/ci/Mac10.14%20Tests/8525

suspect CL:2418018

TBR=

Bug: 1130533
Change-Id: I2f3d46934ecb28a477d888df4422838a706d1b2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421744Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809264}

third_party/blink/web_tests/TestExpectations

@@ -6749,3 +6749,6 @@ crbug.com/1130500 [ Mac10.13 Debug ] virtual/omt-worker-fetch/external/wpt/xhr/x
 
 # Sheriff 2020-09-22
 crbug.com/1130533 [ Mac ] 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.worker.html [ Pass Failure ]