Replace NULL with nullptr or null in webrtc/base/.

webrtc/base/testechoserver.h

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 25 matching lines @@
     for (ClientList::iterator it = client_sockets_.begin();
          it != client_sockets_.end(); ++it) {
       delete *it;
     }
   }
 
   SocketAddress address() const { return server_socket_->GetLocalAddress(); }
 
  private:
   void OnAccept(AsyncSocket* socket) {
-    AsyncSocket* raw_socket = socket->Accept(NULL);
+    AsyncSocket* raw_socket = socket->Accept(nullptr);
     if (raw_socket) {
       AsyncTCPSocket* packet_socket = new AsyncTCPSocket(raw_socket, false);
       packet_socket->SignalReadPacket.connect(this, &TestEchoServer::OnPacket);
       packet_socket->SignalClose.connect(this, &TestEchoServer::OnClose);
       client_sockets_.push_back(packet_socket);
     }
   }
   void OnPacket(AsyncPacketSocket* socket, const char* buf, size_t size,
                 const SocketAddress& remote_addr,
                 const PacketTime& packet_time) {
     rtc::PacketOptions options;
     socket->Send(buf, size, options);
   }
   void OnClose(AsyncPacketSocket* socket, int err) {
     ClientList::iterator it =
         std::find(client_sockets_.begin(), client_sockets_.end(), socket);
     client_sockets_.erase(it);
     Thread::Current()->Dispose(socket);
   }
 
   typedef std::list<AsyncTCPSocket*> ClientList;
   std::unique_ptr<AsyncSocket> server_socket_;
   ClientList client_sockets_;
   RTC_DISALLOW_COPY_AND_ASSIGN(TestEchoServer);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_TESTECHOSERVER_H_