Disable virtual test also crashing with same reason.

before-unload-return-value-from-listener.html is also crashing on the
virtual test suite virtual/mouseevent_fractional with the same reason.

TBR=yutak@chromium.org

Bug: 906952
Change-Id: Ie778939ed40f13d6e1b6e263a6c3266a9427826f
Reviewed-on: https://chromium-review.googlesource.com/c/1358915Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613146}

third_party/blink/web_tests/TestExpectations

@@ -5725,6 +5725,7 @@ crbug.com/905827 [ Mac10.13 ] fast/dom/StyleSheet/stylesheet-move-between-docume
 # Flaky crash due to "bad mojo message".
 crbug.com/906952 editing/pasteboard/file-drag-to-editable.html [ Pass Crash ]
 crbug.com/906952 fast/events/before-unload-return-value-from-listener.html [ Pass Crash ]
+crbug.com/906952 virtual/mouseevent_fractional/fast/events/before-unload-return-value-from-listener.html [ Pass Crash ]
 
 #Sheriff 2018-11-21