[Flaky] Disable flaky vertical-scroll-disabled-scrollbar-tentative.html

The disabled test flakes for 12h with a high frequency.
According to the flakiness dashboard, it affects all platforms:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests&tests=virtual%2Fthreaded%2Fexternal%2Fwpt%2Ffeature-policy%2Fexperimental-features%2Fvertical-scroll-disabled-scrollbar-tentative.html

TBR: iclelland@chromium.org
Bug: 1120106
Change-Id: Ia4bf0c4ac0428cac22ded67a562b4179ee7b9e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367949
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800565}

third_party/blink/web_tests/TestExpectations

@@ -6701,3 +6701,7 @@ crbug.com/1069546 [ Mac ] compositing/layer-creation/overflow-scroll-overlap.htm
 
 # Unexpected demuxer success after M86 FFmpeg roll.
 crbug.com/1117613 media/video-error-networkState.html [ Failure Timeout ]
+
+# Sheriff 2020-0-21
+crbug.com/1120330 virtual/threaded/external/wpt/feature-policy/experimental-features/vertical-scroll-disabled-scrollbar-tentative.html [ Pass Failure ]
+