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

extensions/browser/app_window/app_window_client.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/browser/app_window/app_window_client.h"
 
 #include "base/basictypes.h"
 
 namespace extensions {
 
 namespace {
 
-AppWindowClient* g_client = NULL;
+AppWindowClient* g_client = nullptr;
 
 }  // namespace
 
 AppWindowClient* AppWindowClient::Get() {
   return g_client;
 }
 
 void AppWindowClient::Set(AppWindowClient* client) {
   // This can happen in unit tests, where the utility thread runs in-process.
   if (g_client)
     return;
 
   g_client = client;
 }
 
 }  // namespace extensions