Disable flaky vertical-scroll-wheel-block-manual test.

Flakily timing out.

BUG=853852
TBR=ekaramad
NOTRY=true

Change-Id: I894fe217109a63e86ed5aa2b2be2c801b8f91d76
Reviewed-on: https://chromium-review.googlesource.com/1104977Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568113}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4745,3 +4745,6 @@ crbug.com/853360 [ Mac ] fast/text/drawBidiText.html [ Failure ]
 crbug.com/853360 [ Mac ] fast/forms/calendar-picker/calendar-picker-appearance.html [ Failure ]
 crbug.com/853360 [ Mac ] fast/forms/calendar-picker/month-picker-appearance.html [ Failure ]
 crbug.com/853360 [ Mac ] fast/forms/select/menulist-appearance-rtl.html [ Failure ]
+
+# Sheriff 2018-06-18
+crbug.com/853852 [ Linux ] virtual/threaded/external/wpt/feature-policy/experimental-features/vertical-scroll-wheel-block-manual.tentative.html [ Pass Timeout ]