Reland: Keep poll intervals in prefs.

Cause for breakage: Side effect in DCHECK() which wasn't exercised on trybots
(apparently they have all DCHECKs enabled)

Without persisting them, server-side provided intervals are less efficient on
platforms with relatively short app lifetime.

This also allows lower poll intervals for latency-sensitive use cases in cases
when our notifications are not reliable enough (yet).

Bug: 832019
Change-Id: Ifcd55edcbb9fb17583caac847f6c110024a83e19
Reviewed-on: https://chromium-review.googlesource.com/1013457
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550984}
30 files changed