Mark http/tests/security/redirect-BLOCKED-to-localURL.html as flaky on XP.

TBR=yurys@chromium.org
BUG=311812

Review URL: https://codereview.chromium.org/48633005

git-svn-id: svn://svn.chromium.org/blink/trunk@160847 bbb929c8-8fbe-4397-9dbb-9b2b20218538