Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/content_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_CONTENT_SUGGESTIONS_PROVIDER_H_
 #define COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 91 matching lines @@
   // synchronously.
   virtual void FetchSuggestionImage(const ContentSuggestion::ID& suggestion_id,
                                     const ImageFetchedCallback& callback) = 0;
 
   // Fetches more suggestions for the given category. The new suggestions
   // will not include any suggestion of the |known_suggestion_ids| sets.
   // The given |callback| is called with these suggestions, along with all
   // existing suggestions.
   virtual void Fetch(const Category& category,
                      const std::set<std::string>& known_suggestion_ids,
-                     FetchingCallback callback) = 0;
+                     const FetchingCallback& callback) = 0;
 
   // Removes history from the specified time range where the URL matches the
   // |filter|. The data removed depends on the provider. Note that the
   // data outside the time range may be deleted, for example suggestions, which
   // are based on history from that time range. Providers should immediately
   // clear any data related to history from the specified time range where the
   // URL matches the |filter|.
   virtual void ClearHistory(
       base::Time begin,
       base::Time end,
@@ skipping 27 matching lines @@
   CategoryFactory* category_factory() const { return category_factory_; }
 
  private:
   Observer* observer_;
   CategoryFactory* category_factory_;
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTIONS_PROVIDER_H_