Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/trace_win.cc

 /*
  *  Copyright (c) 2012 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 72 matching lines @@
   GetDateFormat(LOCALE_SYSTEM_DEFAULT, 0, &sys_time, TEXT("MMM dd yyyy"),
                 sz_date_str, 20);
 
   // Create time string (e.g. 15:32:08)
   GetTimeFormat(LOCALE_SYSTEM_DEFAULT, 0, &sys_time, TEXT("HH':'mm':'ss"),
                 sz_time_str, 20);
 
   sprintf(trace_message, "Local Date: %ls Local Time: %ls", sz_date_str,
           sz_time_str);
 
-  // Include NULL termination (hence + 1).
+  // Include null termination (hence + 1).
   return static_cast<int32_t>(strlen(trace_message) + 1);
 }
 
 }  // namespace webrtc