Expected impact:
- On mobile: Before, we would fetch during startup only if an NTP
  happened to be open, i.e. semi-randomly. Now we don't fetch at
  all. This might decrease tile freshness for the first NTP in some
  cases, but the difference should be minimal.
- On desktop, without experiments: No impact, since this code isn't
  used.
- On desktop, with NTPTilesInInstantService: Before, we'd always fetch
  during startup, after Sync finishes initialization. Like on mobile,
  this might decrease freshness in some cases, but only minimally.

On both mobile and desktop, this will significantly reduce traffic to
the server; see the bug for numbers.

BUG=718845

Review-Url: https://codereview.chromium.org/2866013002
Cr-Commit-Position: refs/heads/master@{#469999}