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

components/ntp_snippets/ntp_snippets_service.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 117 matching lines @@
   // etc...). The URL is the only mandatory value.
   bool LoadFromListValue(const base::ListValue& list);
 
   // TODO(treib): Investigate a better storage, maybe LevelDB or SQLite?
   void LoadSnippetsFromPrefs();
   void StoreSnippetsToPrefs();
 
   void LoadDiscardedSnippetsFromPrefs();
   void StoreDiscardedSnippetsToPrefs();
 
+  std::vector<std::string> GetSnippetHostsFromPrefs() const;
+  void StoreSnippetHostsToPrefs(const std::vector<std::string>& hosts);
+
   bool HasDiscardedSnippet(const GURL& url) const;
 
   void RemoveExpiredSnippets();
 
   PrefService* pref_service_;
 
   suggestions::SuggestionsService* suggestions_service_;
 
   // True if the suggestions are loaded.
   bool loaded_;
@@ skipping 48 matching lines @@
   // Send when the service is shutting down.
   virtual void NTPSnippetsServiceShutdown(NTPSnippetsService* service) = 0;
 
  protected:
   virtual ~NTPSnippetsServiceObserver() {}
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_