Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/physical_web_pages/physical_web_page_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_PHYSICAL_WEB_PAGES_PHYSICAL_WEB_PAGE_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_PHYSICAL_WEB_PAGES_PHYSICAL_WEB_PAGE_SUGGESTIONS_PROVIDER_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 33 matching lines @@
   void OnDisplayableUrlsChanged(const std::vector<UrlInfo>& urls);
 
   // 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;
 
  private:
   void NotifyStatusChanged(CategoryStatus new_status);
 
   CategoryStatus category_status_;
   const Category provided_category_;
 
   DISALLOW_COPY_AND_ASSIGN(PhysicalWebPageSuggestionsProvider);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_PHYSICAL_WEB_PAGES_PHYSICAL_WEB_PAGE_SUGGESTIONS_PROVIDER_H_