Add presubmit check for scoped_ptr usage.

ui/app_list/test/test_search_result.cc

 // Copyright 2014 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 "ui/app_list/test/test_search_result.h"
 
 namespace app_list {
 
 TestSearchResult::TestSearchResult() {
 }
 
 TestSearchResult::~TestSearchResult() {
 }
 
 void TestSearchResult::SetDisplayType(SearchResult::DisplayType type) {
   set_display_type(type);
 }
 
 scoped_ptr<SearchResult> TestSearchResult::Duplicate() {
   NOTREACHED();
-  return scoped_ptr<SearchResult>();
+  return nullptr;
 }
 
 }  // namespace app_list