[NTP Snippets] Re-fetch snippets on ML changes only when the hosts actually change

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 kSnippets[] = "ntp_snippets.snippets";
 const char kDiscardedSnippets[] = "ntp_snippets.discarded_snippets";
 
+const char kSnippetHosts[] = "ntp_snippets.hosts";
+
 }  // namespace prefs
 }  // namespace ntp_snippets