Story 1620 (part 2): Promote LeftNav Signup A/B test to Stable

Moving code related to LeftNav Signup A/B test from beta to
stable.

Change-Id: I35055d975c7923c3daedd180fbb351c9ac37bb7e
Merge "Story 1620 (part 2): Promote LeftNav Signup A/B test to Stable"