Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/logcat_trace_context.cc

 /*
  *  Copyright (c) 2013 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 30 matching lines @@
   }
 }
 
 LogcatTraceContext::LogcatTraceContext() {
   webrtc::Trace::CreateTrace();
   if (webrtc::Trace::SetTraceCallback(this) != 0)
     assert(false);
 }
 
 LogcatTraceContext::~LogcatTraceContext() {
-  if (webrtc::Trace::SetTraceCallback(NULL) != 0)
+  if (webrtc::Trace::SetTraceCallback(nullptr) != 0)
     assert(false);
   webrtc::Trace::ReturnTrace();
 }
 
 void LogcatTraceContext::Print(TraceLevel level,
                                const char* message,
                                int length) {
   __android_log_print(AndroidLogPriorityFromWebRtcLogLevel(level),
                       "WEBRTC", "%.*s", length, message);
 }
 
 }  // namespace webrtc