Support server categories in NTPSnippetsService.

Reland of:
https://crrev.com/330958ddcdcd0084d724571eddaa66f9c364115f

Not clear why tests failed; they passed for me and in the CQ.
Removed one synchronization bit that wasn't really part of the change.

BUG=633613

Review-Url: https://codereview.chromium.org/2279863002
Cr-Commit-Position: refs/heads/master@{#414681}
4 files changed