Set virtual/mouseevent_fractional/fast/events/autoscroll-iframe-no-scrolling.html timeout pass

Similar to https://chromium-review.googlesource.com/c/chromium/src/+/1173437
this patch updates autoscroll-iframe-no-scrolling.html to have timeout pass as
expectations.

TBR=hayato@chromium.org

Bug: 873829
Change-Id: Ib534bc8d45b487888c6f75033241786aba768878
Reviewed-on: https://chromium-review.googlesource.com/1183461Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584760}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5005,3 +5005,6 @@ crbug.com/872535 external/wpt/wake-lock/wakelock-state-is-global.https.html [ Pa
 
 # Sheriff 2018-08-20
 crbug.com/862589 [ Linux Mac Win ] virtual/threaded/fast/idle-callback/long_idle_periods.html [ Timeout Pass ]
+
+# Sheriff 2018-08-21
+crbug.com/873829 [ Mac ] virtual/mouseevent_fractional/fast/events/autoscroll-iframe-no-scrolling.html [ Timeout Pass ]