Mark fast/events/middleClickAutoscroll-in-iframe.html timeout on Mac

TBR=wjmaclean@chromium.org
NOTRY=true

Bug: 873435
Change-Id: I2c68d19a86abddf00a3420850fd03a59acb26647
Reviewed-on: https://chromium-review.googlesource.com/1175695
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583223}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3917,6 +3917,9 @@ crbug.com/873454 css3/filters/effect-reference-image-hw.html [ Failure Pass ]
 # Sheriff failures 2018-08-14
 crbug.com/873435 virtual/user-activation-v2/fast/events/middleClickAutoscroll-in-iframe.html [ Timeout Pass ]
 
+# Sheriff failures 2018-08-15
+crbug.com/873435 [ Mac ] fast/events/middleClickAutoscroll-in-iframe.html [ Timeout Pass ]
+
 crbug.com/715718 external/wpt/media-source/mediasource-activesourcebuffers.html [ Failure Pass ]
 crbug.com/715718 external/wpt/media-source/mediasource-remove.html [ Failure Pass ]
 crbug.com/715718 [ Win ] external/wpt/xhr/FormData-append.html [ Failure Pass ]