[sheriff] Disable worklet-animation-with-scroll-timeline-root-scroller.https.html on Mac

TBR=flackr@chromium.org
NOTRY=true

Bug: 938780
Change-Id: I7cdcb81b8801cfafd19a1e9a56616ba0b85e5ace
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504480Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638019}

third_party/blink/web_tests/TestExpectations

@@ -5998,3 +5998,4 @@ crbug.com/937991 [ Win7 Release ] http/tests/devtools/cache-storage/cache-data.j
 
 # Sheriff 2019-03-05
 crbug.com/938200 http/tests/devtools/network/network-blocked-reason.js [ Timeout Pass ] http/tests/devtools/network/network-blocked-reason.js
+crbug.com/938780 [ Mac ] external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html [ Failure ]