[NTP Snippets] Don't reschedule background fetching on every startup

components/ntp_snippets/pref_names.cc

 // Copyright 2016 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.
 
 #include "components/ntp_snippets/pref_names.h"
 
 namespace ntp_snippets {
 namespace prefs {
 
 const char kEnableSnippets[] = "ntp_snippets.enable";
 
 const char kSnippetHosts[] = "ntp_snippets.hosts";
 
+const char kSnippetBackgroundFetchingIntervalWifi[] =
+    "ntp_snippets.fetching_interval_wifi";
+
+const char kSnippetBackgroundFetchingIntervalFallback[] =
+    "ntp_snippets.fetching_interval_fallback";
+
 const char kSnippetFetcherRequestCount[] =
     "ntp.request_throttler.suggestion_fetcher.count";
 const char kSnippetFetcherInteractiveRequestCount[] =
     "ntp.request_throttler.suggestion_fetcher.interactive_count";
 const char kSnippetFetcherRequestsDay[] =
     "ntp.request_throttler.suggestion_fetcher.day";
 
 const char kSnippetThumbnailsRequestCount[] =
     "ntp.request_throttler.suggestion_thumbnails.count";
 const char kSnippetThumbnailsInteractiveRequestCount[] =
@@ skipping 20 matching lines @@
 
 const char kUserClassifierLastTimeToOpenNTP[] =
     "ntp_suggestions.user_classifier.last_time_to_open_ntp";
 const char kUserClassifierLastTimeToShowSuggestions[] =
     "ntp_suggestions.user_classifier.last_time_to_show_suggestions";
 const char kUserClassifierLastTimeToUseSuggestions[] =
     "ntp_suggestions.user_classifier.last_time_to_use_suggestions";
 
 }  // namespace prefs
 }  // namespace ntp_snippets