HHVM beta feature: just manage the cookie for now

Per discussion with Tim, Rob, and Greg on IRC, we should defer UX changes, and
make the beta feature just manage the cookie for now.

Change-Id: Icaf9c1440c72bbc4a4e51a2575f24c0f2c6fb6ce
(cherry picked from commit 86bfc4f45b5baf445717e2875656efd332eca4da)