Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/sessions/foreign_sessions_suggestions_provider.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_SESSIONS_FOREIGN_SESSIONS_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_SESSIONS_FOREIGN_SESSIONS_SUGGESTIONS_PROVIDER_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 41 matching lines @@
   struct SessionData;
 
   // ContentSuggestionsProvider implementation.
   CategoryStatus GetCategoryStatus(Category category) override;
   CategoryInfo GetCategoryInfo(Category category) override;
   void DismissSuggestion(const ContentSuggestion::ID& suggestion_id) override;
   void FetchSuggestionImage(const ContentSuggestion::ID& suggestion_id,
                             const ImageFetchedCallback& callback) override;
   void Fetch(const Category& category,
              const std::set<std::string>& known_suggestion_ids,
-             FetchingCallback callback) override;
+             const FetchingCallback& callback) override;
   void ClearHistory(
       base::Time begin,
       base::Time end,
       const base::Callback<bool(const GURL& url)>& filter) override;
   void ClearCachedSuggestions(Category category) override;
   void GetDismissedSuggestionsForDebugging(
       Category category,
       const DismissedSuggestionsCallback& callback) override;
   void ClearDismissedSuggestionsForDebugging(Category category) override;
 
   void OnForeignTabChange();
   std::vector<ContentSuggestion> BuildSuggestions();
   std::vector<SessionData> GetSuggestionCandidates();
   ContentSuggestion BuildSuggestion(const SessionData& data);
 
   CategoryStatus category_status_;
   const Category provided_category_;
   std::unique_ptr<ForeignSessionsProvider> foreign_sessions_provider_;
   PrefService* pref_service_;
 
   DISALLOW_COPY_AND_ASSIGN(ForeignSessionsSuggestionsProvider);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_SESSIONS_FOREIGN_SESSIONS_SUGGESTIONS_PROVIDER_H_