Run clang-modernize -use-nullptr over src/extensions/.

extensions/shell/browser/desktop_controller.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 "extensions/shell/browser/desktop_controller.h"
 
 #include "base/logging.h"
 #include "base/macros.h"
 
 namespace extensions {
 namespace {
 
-DesktopController* g_instance = NULL;
+DesktopController* g_instance = nullptr;
 
 }  // namespace
 
 // static
 DesktopController* DesktopController::instance() {
   return g_instance;
 }
 
 DesktopController::DesktopController() {
   CHECK(!g_instance) << "DesktopController already exists";
   g_instance = this;
 }
 
 DesktopController::~DesktopController() {
   DCHECK(g_instance);
-  g_instance = NULL;
+  g_instance = nullptr;
 }
 
 }  // namespace extensions