Disable flaky layout test gesture-scrollbar-touchpad-fling.html

Bug: 899222
Change-Id: I4b87bc05b9333f74bd7c2286f8e2a913732ac6ce
Tbr: sahel@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1301565Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603098}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5486,3 +5486,4 @@ crbug.com/874576 http/tests/images/feature-policy-image-compression-cached-image
 
 # Sheriff 2018-10-26
 crbug.com/899087 [ Linux ] virtual/android/fullscreen/full-screen-iframe-allowed-video.html [ Failure Pass ]
+crbug.com/899232 [ Linux ] virtual/user-activation-v2/fast/events/touch/gesture/gesture-scrollbar-touchpad-fling.html [ Failure Pass ]