sheriff: mark middleClickAutoscroll-click flaky on Mac

TBR=pkasting@chromium.org

Bug: 872705
Change-Id: I72edb9f5b1d07baf898fe8aa97231df5d76424a4
Reviewed-on: https://chromium-review.googlesource.com/1171106Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582228}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4900,3 +4900,4 @@ crbug.com/872705 [ Mac ] virtual/mouseevent_fractional/fast/events/recorded-keyd
 
 # Sheriff 2018-08-10
 crbug.com/873078 [ Win7 ] device_orientation/motion/detached-frame.html [ Failure Pass ]
+crbug.com/872705 [ Mac ] virtual/mouseevent_fractional/fast/events/middleClickAutoscroll-click.html [ Timeout Pass ]