Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/test/RTPFile.cc

Index: webrtc/modules/audio_coding/test/RTPFile.cc
diff --git a/webrtc/modules/audio_coding/test/RTPFile.cc b/webrtc/modules/audio_coding/test/RTPFile.cc
index 4bf4c9820af50c7c476cf064ef39f31c3646d887..29aeaaa473d1bda37eb422c4f7cc2746ebcc5c11 100644
--- a/webrtc/modules/audio_coding/test/RTPFile.cc
+++ b/webrtc/modules/audio_coding/test/RTPFile.cc
@@ -122,7 +122,7 @@ bool RTPBuffer::EndOfFile() const {
 }
 
 void RTPFile::Open(const char *filename, const char *mode) {
-  if ((_rtpFile = fopen(filename, mode)) == NULL) {
+  if ((_rtpFile = fopen(filename, mode)) == nullptr) {
     printf("Cannot write file %s.\n", filename);
     ADD_FAILURE() << "Unable to write file";
     exit(1);
@@ -130,9 +130,9 @@ void RTPFile::Open(const char *filename, const char *mode) {
 }
 
 void RTPFile::Close() {
-  if (_rtpFile != NULL) {
+  if (_rtpFile != nullptr) {
     fclose(_rtpFile);
-    _rtpFile = NULL;
+    _rtpFile = nullptr;
   }
 }