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

extensions/browser/extensions_browser_client.cc

 // Copyright 2013 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/extensions_browser_client.h"
 
 #include "base/basictypes.h"
 
 namespace extensions {
 
 namespace {
 
-ExtensionsBrowserClient* g_client = NULL;
+ExtensionsBrowserClient* g_client = nullptr;
 
 }  // namespace
 
 ExtensionsBrowserClient* ExtensionsBrowserClient::Get() {
   return g_client;
 }
 
 void ExtensionsBrowserClient::Set(ExtensionsBrowserClient* client) {
   g_client = client;
 }
 
 }  // namespace extensions