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

components/ntp_snippets/ntp_snippets_service.h

Index: components/ntp_snippets/ntp_snippets_service.h
diff --git a/components/ntp_snippets/ntp_snippets_service.h b/components/ntp_snippets/ntp_snippets_service.h
index ff2b039c00bb6771a4bb3bfab7e67d953349d1ba..4b2f20aa7e4ca2fe220e61fe4ed623ddddfc549b 100644
--- a/components/ntp_snippets/ntp_snippets_service.h
+++ b/components/ntp_snippets/ntp_snippets_service.h
@@ -7,6 +7,7 @@
 
 #include <stddef.h>
 
+#include <set>
 #include <string>
 #include <vector>
 
@@ -118,7 +119,7 @@ class NTPSnippetsService : public KeyedService {
                     scoped_ptr<base::Value> parsed);
   void OnJsonError(const std::string& snippets_json, const std::string& error);
 
-  void FetchSnippetsImpl(const std::vector<std::string>& hosts);
+  void FetchSnippetsImpl(const std::set<std::string>& hosts);
 
   // Expects a top-level dictionary containing a "recos" list, which will be
   // passed to LoadFromListValue().
@@ -136,6 +137,9 @@ class NTPSnippetsService : public KeyedService {
   void LoadDiscardedSnippetsFromPrefs();
   void StoreDiscardedSnippetsToPrefs();
 
+  std::set<std::string> GetSnippetHostsFromPrefs() const;
+  void StoreSnippetHostsToPrefs(const std::set<std::string>& hosts);
+
   bool HasDiscardedSnippet(const GURL& url) const;
 
   void RemoveExpiredSnippets();