Re-enable month-picker-appearance-step.html for Mac

I submitted a change at
https://chromium-review.googlesource.com/c/chromium/src/+/2339377
to fix the mac-retina failure of this test.  The test was disabled
slightly after the fix went in.  The test had started passing after my
fix, e.g. see
https://ci.chromium.org/p/chromium/builders/ci/WebKit%20Mac10.13%20%28retina%29/32112
whose head commit 322be03c contained
the fix but not the disable.

So, re-enabling the test.

TBR=masonfreed@chromium.org,rdevlin.cronin@chromium.org

Bug: 1112111
Change-Id: I090525527448154493f2a74a126d1eff00f882a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339762Reviewed-by: Dan Clark <daniec@microsoft.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#795243}

third_party/blink/web_tests/TestExpectations

@@ -6842,4 +6842,3 @@ 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 Crash ]
-crbug.com/1112111 [ Mac ] fast/forms/month/month-picker-appearance-step.html [ Pass Failure ]