Mark flaky test autoscroll-over-scrollbar.html flaky

Sometimes causes Mac10.10 testing to fail.

TBR=sahel@chromium.org

Bug: 872242
Change-Id: I15b712b498c7398999a21b34c7406a163181cf2e
Reviewed-on: https://chromium-review.googlesource.com/1167048Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581524}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4869,3 +4869,4 @@ crbug.com/851090 [ Mac ] virtual/user-activation-v2/fast/events/autoscroll-over-
 crbug.com/871105 [ Linux ] svg/dom/remove-use-target-element-indirectly.html [ Pass Crash ]
 crbug.com/872025 [ Mac Linux ] fast/css-grid-layout/crash-large-positions.html [ Pass Timeout ]
 crbug.com/871416 fast/spatial-navigation/snav-stay-in-overflow-div.html [ Pass Failure ]
+crbug.com/872242 [ Mac10.10 ] fast/events/autoscroll-over-scrollbar.html [ Pass Failure ]