Reland [Sync] Refactoring polling to be reliable.

Reland [Sync] Refactoring polling to be reliable.

This is a reland of https://codereview.chromium.org/1132013004/

Polling is now an important component of sync health, as it can sometimes be
the only time we'll query for certain datatypes. As such, the following
has changed:
- Polls that fail will be retried (with backoff).
- As such, the logic to force a poll after an auth error isn't needed anymore
- The last successful poll time will be persisted in the sync prefs.
- On startup, schedule the first poll for last_poll_time + poll_interval
  (or Now(), whichever is latest).
- Receiving a new poll interval from the server will update the poll timer
- The poll timer is now a one shot timer, and only restarts on success
- Some code cleanup to make the above more straightforward

BUG=482154

Review URL: https://codereview.chromium.org/1144543004

Cr-Commit-Position: refs/heads/master@{#330024}