Unskip 'http/tests/security/cross-origin-css.html'

It's timed out once on Win7 in the last 3 days, and not at all on Win7
debug or WinXP. That doesn't seem like enough of a problem to accept
skipping the test on Windows.

BUG=111774

Review URL: https://chromiumcodereview.appspot.com/19153003

git-svn-id: svn://svn.chromium.org/blink/trunk@154219 bbb929c8-8fbe-4397-9dbb-9b2b20218538