[Build Sheriff] Disable flaky test on certain Mac builds.

Disables Blink test:
external/wpt/pointerevents/compat/pointerevent_mouse-pointer-preventdefault.html

No-Tree-Checks: true
TBR: liviutinta@chromium.org
Bug: 1124378
Change-Id: I295122b3f5136cc03eada8541b32ead3121c346b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391386
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803987}

third_party/blink/web_tests/TestExpectations

@@ -2552,8 +2552,9 @@ crbug.com/1105958 external/wpt/payment-request/payment-is-showing.https.html [ T
 
 # ====== New tests from wpt-importer added here ======
 crbug.com/626703 external/wpt/css/css-pseudo/file-selector-button-001.tentative.html [ Failure ]
-crbug.com/626703 [ Mac10.13 ] external/wpt/pointerevents/compat/pointerevent_mouse-pointer-preventdefault.html [ Timeout ]
+crbug.com/626703 [ Mac10.13 ] external/wpt/pointerevents/compat/pointerevent_mouse-pointer-preventdefault.html [ Timeout Pass Failure ]
 crbug.com/626703 [ Mac10.14 ] external/wpt/pointerevents/compat/pointerevent_mouse-pointer-preventdefault.html [ Timeout ]
+crbug.com/626703 [ Mac10.15 ] external/wpt/pointerevents/compat/pointerevent_mouse-pointer-preventdefault.html [ Pass Failure ]
 crbug.com/626703 [ Mac10.15 ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-root-scroller.https.html [ Failure ]
 crbug.com/626703 [ Mac10.14 ] virtual/off-main-thread-css-paint/external/wpt/css/css-paint-api/valid-image-before-load.https.html [ Failure ]
 crbug.com/626703 external/wpt/web-share/share-url-invalid.https.html [ Crash ]