Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/remote/ntp_snippets_service.cc

 // Copyright 2015 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/remote/ntp_snippets_service.h"
 
 #include <algorithm>
 #include <iterator>
 #include <utility>
 
@@ skipping 313 matching lines @@
   params.hosts = hosts;
   params.interactive_request = interactive_request;
   snippets_fetcher_->FetchSnippets(
       params, base::BindOnce(&NTPSnippetsService::OnFetchFinished,
                              base::Unretained(this)));
 }
 
 void NTPSnippetsService::Fetch(
     const Category& category,
     const std::set<std::string>& known_suggestion_ids,
-    FetchingCallback callback) {
+    const FetchingCallback& callback) {
   if (!ready()) {
     CallWithEmptyResults(callback);
     return;
   }
   NTPSnippetsFetcher::Params params =
       BuildFetchParams(/*exclude_archived_suggestions=*/false);
   params.excluded_ids.insert(known_suggestion_ids.begin(),
                              known_suggestion_ids.end());
   params.interactive_request = true;
   params.exclusive_category = category;
@@ skipping 892 matching lines @@
 }
 
 NTPSnippetsService::CategoryContent::CategoryContent() = default;
 NTPSnippetsService::CategoryContent::CategoryContent(CategoryContent&&) =
     default;
 NTPSnippetsService::CategoryContent::~CategoryContent() = default;
 NTPSnippetsService::CategoryContent& NTPSnippetsService::CategoryContent::
 operator=(CategoryContent&&) = default;
 
 }  // namespace ntp_snippets