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

components/ntp_snippets/pref_names.cc

Index: components/ntp_snippets/pref_names.cc
diff --git a/components/ntp_snippets/pref_names.cc b/components/ntp_snippets/pref_names.cc
index 597a9c3816c50eba9dd3526ce483cc7f5cde39ef..a7b9f888b0e713d525f3c8653e9bd2398cca497d 100644
--- a/components/ntp_snippets/pref_names.cc
+++ b/components/ntp_snippets/pref_names.cc
@@ -10,5 +10,7 @@ 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