Mark before-unload-return-bad-value as timing out on debug too.

This test has been observed timing out in debug builds as
well as release builds.
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.6%20%28dbg%29/builds/16950

BUG=398306
TBR=haraken@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/454453002

git-svn-id: svn://svn.chromium.org/blink/trunk@179735 bbb929c8-8fbe-4397-9dbb-9b2b20218538