Unskip and rebaseline 'http/tests/security/sandboxed-iframe-form-top.html'

After contentshell, it doesn't appear that this test is leaking state into
the next test anymore. Yay!

R=jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19167003

git-svn-id: svn://svn.chromium.org/blink/trunk@154229 bbb929c8-8fbe-4397-9dbb-9b2b20218538