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

webrtc/base/testechoserver.h

Index: webrtc/base/testechoserver.h
diff --git a/webrtc/base/testechoserver.h b/webrtc/base/testechoserver.h
index 76d714b4f4a10300278fb1a95eab3f05abbe21c7..97606c105cd2d893d3339867e3abe8a7ee3338ee 100644
--- a/webrtc/base/testechoserver.h
+++ b/webrtc/base/testechoserver.h
@@ -43,7 +43,7 @@ class TestEchoServer : public sigslot::has_slots<> {
 
  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);