Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/field_trial_default.cc

Index: webrtc/system_wrappers/source/field_trial_default.cc
diff --git a/webrtc/system_wrappers/source/field_trial_default.cc b/webrtc/system_wrappers/source/field_trial_default.cc
index 0e2c286117a1a2cc96687c61b5f6c2483b0e4c37..acd3cf742686cb1c2799d6f770bd2c0be53ef99b 100644
--- a/webrtc/system_wrappers/source/field_trial_default.cc
+++ b/webrtc/system_wrappers/source/field_trial_default.cc
@@ -17,10 +17,10 @@
 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);