Mark 'fast/events/middleClickAutoscroll-drag.html' to be flaky as well.

TBR=wjmaclean@chromium.org

Bug: 891155
Change-Id: I27212eee19d0f7f67b4ee5caba3df4562edb30d2
Reviewed-on: https://chromium-review.googlesource.com/c/1281225Reviewed-by: Chan Li <chanli@chromium.org>
Commit-Queue: Chan Li <chanli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599787}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5325,4 +5325,5 @@ crbug.com/891155 [ Linux ] virtual/user-activation-v2/fast/events/middleClickAut
 
 # Sheriff 2018-10-15
 crbug.com/895257 [ Mac ] external/wpt/css/css-fonts/variations/at-font-face-font-matching.html [ Failure Pass ]
-crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-event-fired.html [ Failure Pass ]
\ No newline at end of file
+crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-event-fired.html [ Failure Pass ]
+crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-drag.html [ Failure Pass ]