Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/offline_pages/recent_tab_suggestions_provider.cc

 // 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.
 
 #include "components/ntp_snippets/offline_pages/recent_tab_suggestions_provider.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 88 matching lines @@
     const ImageFetchedCallback& callback) {
   // TODO(vitaliii): Fetch proper thumbnail from OfflinePageModel once it's
   // available there.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, gfx::Image()));
 }
 
 void RecentTabSuggestionsProvider::Fetch(
     const Category& category,
     const std::set<std::string>& known_suggestion_ids,
-    FetchingCallback callback) {
+    const FetchingCallback& callback) {
   NOTREACHED();
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(callback, base::Passed(std::vector<ContentSuggestion>())));
 }
 
 void RecentTabSuggestionsProvider::ClearHistory(
     base::Time begin,
     base::Time end,
     const base::Callback<bool(const GURL& url)>& filter) {
@@ skipping 154 matching lines @@
 }
 
 void RecentTabSuggestionsProvider::StoreDismissedIDsToPrefs(
     const std::set<std::string>& dismissed_ids) {
   prefs::StoreDismissedIDsToPrefs(pref_service_,
                                   prefs::kDismissedRecentOfflineTabSuggestions,
                                   dismissed_ids);
 }
 
 }  // namespace ntp_snippets