[AnimationWorklet] Disable flaky test on Mac

virtual/threaded/external/wpt/animation-worklet/stateful-animator.https.html
is still flaky on Mac [1].
[1] https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual/threaded/external/wpt/animation-worklet/stateful-animator.https.html

TBR=yigu@chromium.org
NOTRY=true

Bug: 1014810, 1014812
Change-Id: Ib6811c0f14fbfcecff0d5d151ae446afbda5e98d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890615Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710755}

third_party/blink/web_tests/TestExpectations

@@ -5746,6 +5746,7 @@ crbug.com/1018064 [ Mac10.13 ] virtual/threaded/http/tests/devtools/tracing/time
 
 # Sheriff 2019-10-30
 crbug.com/1018027 virtual/threaded-no-composited-antialiasing/animations/custom-properties/registered-var-to-registered.html [ Pass Crash ]
+crbug.com/1014810 [ Mac ] virtual/threaded/external/wpt/animation-worklet/stateful-animator.https.html [ Pass Timeout ]
 
 # Temporarily disabled for landing coverage changes in DevTools
 crbug.com/1009396 http/tests/devtools/coverage/coverage-export.js [ Pass Failure ]