Mark flaky test as skip on Win due to timeout

Test external/wpt/feature-policy/experimental-features/vertical-scroll-touch-block-manual.tentative.html
has been marked as flaky by Try-Flakes, skipping until further
investigation.

Bug: 850964
Tbr: ekaramad@chromium.org
Change-Id: I13ef5fd0520a2e04e1c7166fff2fd2a52a8d0add
Reviewed-on: https://chromium-review.googlesource.com/1123739Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572083}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2368,8 +2368,8 @@ crbug.com/613672 [ Mac ] virtual/user-activation-v2/fast/events/pointerevents/po
 crbug.com/613672 [ Mac ] virtual/threaded/fast/events/pointerevents/pinch/pointerevent_touch-action-pinch_zoom_touch.html [ Skip ]
 crbug.com/613672 [ Mac ] external/wpt/pointerevents/compat/pointerevent_touch-action_two-finger_interaction-manual.html [ Skip ]
 # In addition to having no support on Mac, the following test times out
-# regularly on Windows.
-crbug.com/613672 [ Mac ] virtual/threaded/external/wpt/feature-policy/experimental-features/vertical-scroll-touch-block-manual.tentative.html [ Skip ]
+# regularly on Windows. crbug.com/850964
+crbug.com/613672 [ Mac Win ] virtual/threaded/external/wpt/feature-policy/experimental-features/vertical-scroll-touch-block-manual.tentative.html [ Skip ]
 crbug.com/613672 [ Mac ] fast/events/pointerevents/multi-touch-events.html [ Skip ]
 crbug.com/613672 [ Mac ] virtual/user-activation-v2/fast/events/pointerevents/multi-touch-events.html [ Skip ]
 crbug.com/613672 [ Mac ] virtual/mouseevent_fractional/fast/events/pointerevents/multi-touch-events.html [ Skip ]