Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/sessions/foreign_sessions_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/sessions/foreign_sessions_suggestions_provider.h"
 
 #include <algorithm>
 #include <map>
 #include <tuple>
 #include <utility>
@@ skipping 136 matching lines @@
 void ForeignSessionsSuggestionsProvider::FetchSuggestionImage(
     const ContentSuggestion::ID& suggestion_id,
     const ImageFetchedCallback& callback) {
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, gfx::Image()));
 }
 
 void ForeignSessionsSuggestionsProvider::Fetch(
     const Category& category,
     const std::set<std::string>& known_suggestion_ids,
-    FetchingCallback callback) {
+    const FetchingCallback& callback) {
   NOTREACHED();
 }
 
 void ForeignSessionsSuggestionsProvider::ClearHistory(
     base::Time begin,
     base::Time end,
     const base::Callback<bool(const GURL& url)>& filter) {
   std::set<std::string> dismissed_ids = prefs::ReadDismissedIDsFromPrefs(
       *pref_service_, prefs::kDismissedForeignSessionsSuggestions);
   for (auto iter = dismissed_ids.begin(); iter != dismissed_ids.end();) {
@@ skipping 150 matching lines @@
   // This field is sandwiched between the url's favicon, which is on the left,
   // and the |publish_date|, which is to the right. The domain should always
   // appear next to the favicon.
   suggestion.set_publisher_name(
       base::UTF8ToUTF16(data.navigation->virtual_url().host() + " - " +
                         data.session->session_name));
   return suggestion;
 }
 
 }  // namespace ntp_snippets