[WPT][Flaky] Mark before-unload-return-value-from-listener.html flaky

Like other before-unload-return-value-from-listener.html, this one also
crashes flakily with the "bad mojo message" reason.

It also has a history of flakiness:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fuser-activation-v2%2Ffast%2Fevents%2Fbefore-unload-return-value-from-listener.html&testType=webkit_layout_tests

TBR=yutak@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 906952
Change-Id: I13b618385de5a49f83638229b05454ef38092fad
Reviewed-on: https://chromium-review.googlesource.com/c/1373762
Reviewed-by: Friedrich Horschig [CET] <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615879}
1 file changed