Mark pointerevent_touch-action-pinch_zoom_touch.html flaky on Mac

TBR=bokan@chromium.org

See dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Fthreaded%2Ffast%2Fevents%2Fpointerevents%2Fpinch%2Fpointerevent_touch-action-pinch_zoom_touch.html%20

Bug: 893659
Change-Id: I520774abc21e15a2fb3e55b04c7195d36cfb3804
Reviewed-on: https://chromium-review.googlesource.com/c/1496484
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636870}

third_party/blink/web_tests/TestExpectations

@@ -5960,3 +5960,4 @@ crbug.com/937215 [ Linux ] media/controls/modern/doubletap-to-jump-forwards.html
 crbug.com/937215 [ Linux ] virtual/video-surface-layer/media/controls/modern/doubletap-to-jump-forwards.html [ Pass Failure ]
 crbug.com/937170 [ Linux Win7 ] external/wpt/IndexedDB/interleaved-cursors-large.html [ Pass Timeout Crash ]
 crbug.com/937312 [ Win7 ] external/wpt/background-fetch/fetch.https.window.html [ Pass Timeout ]
+crbug.com/893659 [ Mac ] virtual/threaded/fast/events/pointerevents/pinch/pointerevent_touch-action-pinch_zoom_touch.html [ Pass Failure Crash ]