Replace NULL with nullptr in all C++ files.

webrtc/tools/converter/converter.cc

Index: webrtc/tools/converter/converter.cc
diff --git a/webrtc/tools/converter/converter.cc b/webrtc/tools/converter/converter.cc
index a9b453d5091f5680d59af6ff5b9d58e596922994..48b961350a4afe55a089ddf3099d90855e164978 100644
--- a/webrtc/tools/converter/converter.cc
+++ b/webrtc/tools/converter/converter.cc
@@ -38,7 +38,7 @@ bool Converter::ConvertRGBAToI420Video(std::string frames_dir,
   FILE* output_file = fopen(output_file_name.c_str(), "wb");
 
   // Open output file in append mode.
-  if (output_file == NULL) {
+  if (output_file == nullptr) {
     fprintf(stderr, "Couldn't open input file for reading: %s\n",
             output_file_name.c_str());
     return false;
@@ -135,7 +135,7 @@ bool Converter::AddYUVPlaneToFile(uint8_t* yuv_plane,
 bool Converter::ReadRGBAFrame(const char* input_file_name, int input_frame_size,
                               unsigned char* buffer) {
   FILE* input_file = fopen(input_file_name, "rb");
-  if (input_file == NULL) {
+  if (input_file == nullptr) {
     fprintf(stderr, "Couldn't open input file for reading: %s\n",
             input_file_name);
     return false;