[NTP Snippets] Schedule periodic fetching

components/ntp_snippets.gypi

 # Copyright 2015 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.
 
 {
   'targets': [
     {
       # GN version: //components/ntp_snippets
       'target_name': 'ntp_snippets',
       'type': 'static_library',
       'include_dirs': [
         '..',
       ],
       'dependencies': [
         '../base/base.gyp:base',
         '../url/url.gyp:url_lib',
         'keyed_service_core',
       ],
       'sources': [
         'ntp_snippets/inner_iterator.h',
         'ntp_snippets/ntp_snippet.cc',
         'ntp_snippets/ntp_snippet.h',
         'ntp_snippets/ntp_snippets_fetcher.cc',
         'ntp_snippets/ntp_snippets_fetcher.h',
+        'ntp_snippets/ntp_snippets_scheduler.h',
         'ntp_snippets/ntp_snippets_service.cc',
         'ntp_snippets/ntp_snippets_service.h',
       ],
     },
   ],
 }