Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/offline_pages/recent_tab_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_OFFLINE_PAGES_RECENT_TAB_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_OFFLINE_PAGES_RECENT_TAB_SUGGESTIONS_PROVIDER_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 31 matching lines @@
   ~RecentTabSuggestionsProvider() override;
 
   // 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;
 
@@ skipping 50 matching lines @@
   PrefService* pref_service_;
 
   base::WeakPtrFactory<RecentTabSuggestionsProvider> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RecentTabSuggestionsProvider);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_OFFLINE_PAGES_RECENT_TAB_SUGGESTIONS_PROVIDER_H_