Mark a virtual/threaded test as failing on Mac

Example of failed build:
https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/17499

Bug: 1061131

TBR=alancutter@chromium.org

Change-Id: I0b7c61911508bdae5fae1b42531ea06955bfd804
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2101648Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749929}

third_party/blink/web_tests/TestExpectations

@@ -7027,3 +7027,4 @@ crbug.com/1061131 [ Linux ] virtual/threaded-no-composited-antialiasing/animatio
 crbug.com/1061131 [ Linux ] virtual/threaded-no-composited-antialiasing/animations/animationworklet/scroll-timeline-non-scrollable.html [ Pass Crash ]
 crbug.com/1061131 [ Linux ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html [ Pass Crash ]
 crbug.com/1061131 [ Linux ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline.https.html [ Pass Crash ]
+crbug.com/1061131 [ Mac ] virtual/threaded/external/wpt/scroll-animations/two-animations-attach-to-same-scroll-timeline-cancel-one.html [ Pass Failure ]