Mark non-virtual middleClickAutoscroll-click as flaky on Mac.

TBR=pkasting@chromium.org
NOTRY=true (disabling tests)

Bug: 872705
Change-Id: Id68190e7ece3004b58fe1450a9e2421d7fad81da
Reviewed-on: https://chromium-review.googlesource.com/1180251Reviewed-by: apacible <apacible@chromium.org>
Commit-Queue: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584148}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4943,6 +4943,7 @@ crbug.com/872705 [ Mac ] virtual/mouseevent_fractional/fast/events/recorded-keyd
 
 # Sheriff 2018-08-10
 crbug.com/872705 [ Mac ] virtual/mouseevent_fractional/fast/events/middleClickAutoscroll-click.html [ Timeout Pass ]
+crbug.com/872705 [ Mac ] fast/events/middleClickAutoscroll-click.html [ Timeout Pass ]
 
 # Sheriff 2018-08-13
 crbug.com/873829 [ Mac ] virtual/user-activation-v2/fast/events/autoscroll-iframe-no-scrolling.html [ Timeout Pass ]