Mark before-unload-return-bad-value as timing out

TBR=rjwright@chromium.org
NOTRY=true
BUG=39830

Review URL: https://codereview.chromium.org/439523002

git-svn-id: svn://svn.chromium.org/blink/trunk@179409 bbb929c8-8fbe-4397-9dbb-9b2b20218538