[Sync] Stop purging sync data on restart for users who haven't finished setup.

Users who don't have first setup complete, but are signed in without sync
suppressed, are in an invalid state, likely from previous issues in the
sign in flow. We were purging their sync data over-aggressively on startup
previously, causing a resync of the control types on each restart. Now we
simply leave the data around, and only purge if the user is actually signed


Review-Url: https://codereview.chromium.org/2113453006
Cr-Commit-Position: refs/heads/master@{#403382}
2 files changed