[NTP Snippets] Persist the set of snippets in a pref

components/ntp_snippets/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP version: components/ntp_snippets.gypi:ntp_snippets
 source_set("ntp_snippets") {
   sources = [
     "inner_iterator.h",
     "ntp_snippet.cc",
     "ntp_snippet.h",
     "ntp_snippets_fetcher.cc",
     "ntp_snippets_fetcher.h",
+    "ntp_snippets_scheduler.h",
     "ntp_snippets_service.cc",
     "ntp_snippets_service.h",
+    "pref_names.cc",
+    "pref_names.h",
   ]
 
   public_deps = [
     "//base",
     "//components/keyed_service/core",
+    "//components/pref_registry",
+    "//components/prefs",
     "//components/signin/core/browser",
     "//google_apis",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "inner_iterator_unittest.cc",
     "ntp_snippets_service_unittest.cc",
   ]
 
   deps = [
     ":ntp_snippets",
     "//base",
     "//components/signin/core/browser:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }