Set virtual/user-activation-v2/fast/events/autoscroll-iframe-no-scrolling.html timeout pass

TBR: hayato@chromium.org
Bug: 873829
Change-Id: I321d85a63971b1f9b01bb42d4b5d93db44125c92
Reviewed-on: https://chromium-review.googlesource.com/1173437
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582752}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4902,3 +4902,6 @@ 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 ]
+
+# Sheriff 2018-08-13
+crbug.com/873829 [ Mac ] virtual/user-activation-v2/fast/events/autoscroll-iframe-no-scrolling.html [ Timeout Pass ]