Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/field_trial_default.cc

 // Copyright (c) 2014 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.
 //
 
 #include "webrtc/system_wrappers/include/field_trial.h"
 #include "webrtc/system_wrappers/include/field_trial_default.h"
 
 #include <string>
 
 // Simple field trial implementation, which allows client to
 // specify desired flags in InitFieldTrialsFromString.
 namespace webrtc {
 namespace field_trial {
 
-static const char *trials_init_string = NULL;
+static const char* trials_init_string = nullptr;
 
 std::string FindFullName(const std::string& name) {
-  if (trials_init_string == NULL)
+  if (trials_init_string == nullptr)
     return std::string();
 
   std::string trials_string(trials_init_string);
   if (trials_string.empty())
     return std::string();
 
   static const char kPersistentStringSeparator = '/';
   size_t next_item = 0;
   while (next_item < trials_string.length()) {
 
@@ skipping 23 matching lines @@
 void InitFieldTrialsFromString(const char* trials_string) {
   trials_init_string = trials_string;
 }
 
 const char* GetFieldTrialString() {
   return trials_init_string;
 }
 
 }  // namespace field_trial
 }  // namespace webrtc