Mark external/wpt/css/css-fonts/variations/at-font-face-font-matching.html as flaky

TBR=drott@chromium.org

Bug: 895257
Change-Id: I21c5097c48a46ce8cf3843dd179f69d628c23f0d
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1280422Reviewed-by: Florent Castelli <orphis@chromium.org>
Commit-Queue: Florent Castelli <orphis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599599}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5334,3 +5334,6 @@ crbug.com/891155 [ Linux ] virtual/mouseevent_fractional/fast/events/middleClick
 crbug.com/891155 [ Linux ] virtual/mouseevent_fractional/fast/events/middleClickAutoscroll-event-fired.html [ Failure Pass ]
 crbug.com/891155 [ Linux ] virtual/user-activation-v2/fast/events/middleClickAutoscroll-drag.html [ Failure Pass ]
 crbug.com/891155 [ Linux ] virtual/user-activation-v2/fast/events/middleClickAutoscroll-event-fired.html [ Failure Pass ]
+
+# Sheriff 2018-10-15
+crbug.com/895257 [ Mac ] external/wpt/css/css-fonts/variations/at-font-face-font-matching.html [ Failure Pass ]