Fixes an invalidated pointer access in NTPSnipppetsFetcher.

chrome/browser/ntp_snippets/download_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 CHROME_BROWSER_NTP_SNIPPETS_DOWNLOAD_SUGGESTIONS_PROVIDER_H_
 #define CHROME_BROWSER_NTP_SNIPPETS_DOWNLOAD_SUGGESTIONS_PROVIDER_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 45 matching lines @@
       ntp_snippets::Category category) override;
   ntp_snippets::CategoryInfo GetCategoryInfo(
       ntp_snippets::Category category) override;
   void DismissSuggestion(
       const ntp_snippets::ContentSuggestion::ID& suggestion_id) override;
   void FetchSuggestionImage(
       const ntp_snippets::ContentSuggestion::ID& suggestion_id,
       const ImageFetchedCallback& callback) override;
   void Fetch(const ntp_snippets::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(ntp_snippets::Category category) override;
   void GetDismissedSuggestionsForDebugging(
       ntp_snippets::Category category,
       const DismissedSuggestionsCallback& callback) override;
   void ClearDismissedSuggestionsForDebugging(
       ntp_snippets::Category category) override;
@@ skipping 142 matching lines @@
   // Whether the Download Manager UI is enabled, in which case the More button
   // for the Downloads section can redirect there.
   const bool download_manager_ui_enabled_;
 
   base::WeakPtrFactory<DownloadSuggestionsProvider> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadSuggestionsProvider);
 };
 
 #endif  // CHROME_BROWSER_NTP_SNIPPETS_DOWNLOAD_SUGGESTIONS_PROVIDER_H_