[Sheriff] Disable fast/forms/month/month-picker-appearance-step.html

Flaky on Mac.

TBR=rogerta@chromium.org

Bug: 1112111
Change-Id: Id90a9c6333874bb18ec1300537b8df583b21e132
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339499
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795090}

third_party/blink/web_tests/TestExpectations

@@ -6854,4 +6854,5 @@ crbug.com/1112771 external/wpt/css/css-grid/grid-definition/grid-inline-template
 
 # Sheriff 2020-08-05
 crbug.com/1113050 fast/borders/border-radius-mask-video-ratio.html [ Pass Failure ]
-crbug.com/1113127 fast/canvas/downsample-quality.html [ Pass Failure ]
+crbug.com/1113127 fast/canvas/downsample-quality.html [ Pass Failure ]
\ No newline at end of file
+crbug.com/1112111 [ Mac ] fast/forms/month/month-picker-appearance-step.html [ Pass Failure ]