[Animation Worklet] Enable layout test on Mac10.12 onwards

The test is only flaky on Mac10.10 and Mac10.11 [1].

[1] https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests%20(with%20patch)&tests=virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline.https.html

Bug: 938780
Change-Id: I5e30fcd0ebdad632a7e6905ccc4f2edb986ca9ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541589Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644862}

third_party/blink/web_tests/TestExpectations

@@ -6065,7 +6065,7 @@ crbug.com/937991 [ Win7 Release ] virtual/binary-for-devtools/http/tests/devtool
 # Sheriff 2019-03-05
 crbug.com/938200 http/tests/devtools/network/network-blocked-reason.js [ Timeout Pass ]
 crbug.com/938200 virtual/binary-for-devtools/http/tests/devtools/network/network-blocked-reason.js [ Timeout Pass ]
-crbug.com/938780 [ Mac ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html [ Failure ]
+crbug.com/938780 [ Mac10.10 Mac10.11 ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html [ Pass Failure ]
 
 # Sheriff 2019-03-06
 crbug.com/938591 [ Linux ] fast/events/middleClickAutoscroll-nested-divs-forbidden.html [ Failure Pass ]