replace NULL->nullptr in src/remoting.

remoting/host/username.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 "remoting/host/username.h"
 
 #include <vector>
 
 #include "base/logging.h"
 
 #if defined(OS_POSIX)
 #include <pwd.h>
 #include <sys/types.h>
 #include <unistd.h>
 #endif  // defined(OS_POSIX)
 
 namespace remoting {
 
 std::string GetUsername() {
 #if defined(OS_POSIX)
   long buf_size = sysconf(_SC_GETPW_R_SIZE_MAX);
   if (buf_size <= 0)
     return std::string();
 
   std::vector<char> buf(buf_size);
   struct passwd passwd;
-  struct passwd* passwd_result = NULL;
+  struct passwd* passwd_result = nullptr;
   getpwuid_r(getuid(), &passwd, &(buf[0]), buf_size, &passwd_result);
   return passwd_result ? passwd_result->pw_name : std::string();
 #else  // !defined(OS_POSIX)
   NOTIMPLEMENTED();
   return std::string();
 #endif  // defined(OS_POSIX)
 }
 
 }  // namespace remoting