[ScrollTimeline] Mark test no longer flaky

The test does not seem to be flaky [1] after crrev.com/c/2138167.

[1] https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests&tests=virtual%2Fthreaded%2Fexternal%2Fwpt%2Fscroll-animations%2Ftwo-animations-attach-to-same-scroll-timeline-cancel-one.html

Bug: 1060974
Change-Id: I653d199a032678ae3d33be449dc392b8f895acfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2167542Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762713}

third_party/blink/web_tests/TestExpectations

@@ -6584,7 +6584,6 @@ crbug.com/1058785 external/wpt/html/cross-origin-embedder-policy/cache-storage-r
 
 # Sheriff 2020-03-12
 crbug.com/1061131 [ Linux ] virtual/threaded-prefer-compositing/fast/scrolling/middle-click-autoscroll-latching-clicked-node.html [ Pass Failure Timeout ]
-crbug.com/1060974 [ Debug ] virtual/threaded/external/wpt/scroll-animations/two-animations-attach-to-same-scroll-timeline-cancel-one.html [ Pass Failure ]
 
 # Failing on Fuchsia due to dependency on FuchsiaMediaResourceProvider, which is not implemented in content_shell.
 crbug.com/1061226 [ Fuchsia ] fast/mediastream/MediaStream-onactive-oninactive.html [ Crash ]