[Sheriff] Disable fast/events/mouse-cursor-image-set.html.

The test fast/events/mouse-cursor-image-set.html is flaky on Mac 10.12
Tests builder. e.g.
https://ci.chromium.org/p/chromium/builders/ci/Mac10.12%20Tests/38366.

BUG=1137716
TBR=tkent@chromium.org

Change-Id: Ib4d284ed30a1e1968ca02fef85ffe2690fd5c478
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467577Reviewed-by: Maggie Cai <mxcai@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816496}

third_party/blink/web_tests/TestExpectations

@@ -6539,3 +6539,6 @@ crbug.com/1134237 virtual/origin-trials-runtimeflags-disabled/http/tests/origin_
 # Sheriff 2020-10-09
 crbug.com/1136687 external/wpt/pointerevents/pointerlock/pointerevent_pointerlock_supercedes_capture.html [ Pass Failure ]
 crbug.com/1136726 [ Linux ] virtual/gpu-rasterization/images/imagemap-focus-ring-outline-color-not-inherited-from-map.html [ Failure ]
+
+# Sheriff 2020-10-13
+crbug.com/1137716 [ Mac10.12 ] fast/events/mouse-cursor-image-set.html [ Pass Failure ]
\ No newline at end of file