Ignoring middleClickAutoscroll-latching.html timeout on mac.

This test is timing out (flaky 50%) on macOS 10.10 and 10.13.

TBRing rakina@ for review and sunyunjia@ (owner of bug).

TBR=rakina@chromium.org,sunyunjia@chromium.org

Bug: 827209
Change-Id: I2818f917ccc88d9275014ed8971edb6469638856
Reviewed-on: https://chromium-review.googlesource.com/1174901Reviewed-by: Chris Mumford <cmumford@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582983}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4495,7 +4495,8 @@ crbug.com/754986 virtual/video-surface-layer/media/video-transformed.html [ Pass
 crbug.com/827088 bluetooth/requestDevice/chooser/new-scan-device-added.html [ Pass Crash ]
 crbug.com/827209 fast/events/middleClickAutoscroll-latching.html [ Pass Failure ]
 crbug.com/827209 virtual/mouseevent_fractional/fast/events/middleClickAutoscroll-latching.html [ Pass Failure ]
-crbug.com/827209 virtual/user-activation-v2/fast/events/middleClickAutoscroll-latching.html [ Pass Failure ]
+# Sheriff 2018-08-14
+crbug.com/827209 virtual/user-activation-v2/fast/events/middleClickAutoscroll-latching.html [ Pass Timeout Failure ]
 
 # Utility for manual testing, not intended to be run as part of layout tests.
 crbug.com/785955 http/tests/credentialmanager/tools/virtual-authenticator-environment-manual.html [ Skip ]