Disable virtual/threaded/.../worklet-animation-local-time-null-1

Looks like this has been failing flakily on Windows and Mac for a while;
most recently, it's caused the Mac10.13 Tests (dbg) builder to fail:

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29

Tbr: gtsteel@chromium.org
Bug: 1126649
Change-Id: Ib3313cdcee9351d1e2bb1b4553456b2bc6c6e5d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401973
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805593}

third_party/blink/web_tests/TestExpectations

@@ -6640,3 +6640,7 @@ crbug.com/1124978 [ Mac ] virtual/threaded-composited-iframes/external/wpt/is-in
 crbug.com/1122490 [ Mac11.0 ] virtual/text-antialias/mac-system-ui-weight.html [ Failure ]
 crbug.com/1122490 [ Mac11.0 ] virtual/text-antialias/mac-system-ui-trak.html [ Failure ]
 crbug.com/1123002 [ Mac11.0 ] external/wpt/html/dom/elements/global-attributes/dir_auto-textarea-script-N-between-Rs.html [ Failure ]
+
+# Sheriff 2020-09-09
+crbug.com/1126709 [ Mac ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-local-time-null-1.https.html [ Pass Failure ]
+crbug.com/1126709 [ Win ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-local-time-null-1.https.html [ Pass Failure ]