Mark fast/events/before-unload-return-bad-value.html as crashing in debug

Follow up to previous patch: https://codereview.chromium.org/429573003

NOTRY=TRUE
TBR=haraken@chromium.org,ishell@chromium.org
BUG=398306

Review URL: https://codereview.chromium.org/435563002

git-svn-id: svn://svn.chromium.org/blink/trunk@179302 bbb929c8-8fbe-4397-9dbb-9b2b20218538