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
Committed: https://crrev.com/b1133ac9da61c61238ddc112099cc9fbd883853e
Cr-Commit-Position: refs/heads/master@{#330024}

[Nicolas Zea, 2015-05-14 20:53:03]

zea@chromium.org changed reviewers:
+ pavely@chromium.org

[Nicolas Zea, 2015-05-14 20:53:03]

Pavel, could you take a look again? Patchset 1 contains the diff from the
previously landed version. The flaky tests revealed a bug in the code.

[Nicolas Zea, 2015-05-14 22:59:09]

Actually, you can hold off on reviewing. I mixed up the version of the patch I
sent out. Will reupload with a correct version.

[Nicolas Zea, 2015-05-14 23:26:49]

Ok, PTAL again. Patchset 3 has the diff from the previously LGTMd CL.

[pavely, 2015-05-15 00:20:14]

lgtm

[Nicolas Zea, 2015-05-15 00:21:21]

The CQ bit was checked by zea@chromium.org

[commit-bot: I haz the power, 2015-05-15 00:22:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1144543004/60001

[commit-bot: I haz the power, 2015-05-15 02:23:18]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-05-15 02:23:57]

Patchset 4 (id:??) landed as
https://crrev.com/b1133ac9da61c61238ddc112099cc9fbd883853e
Cr-Commit-Position: refs/heads/master@{#330024}