Replace NULL with nullptr in all C++ files.

webrtc/test/rtp_file_writer.cc

Index: webrtc/test/rtp_file_writer.cc
diff --git a/webrtc/test/rtp_file_writer.cc b/webrtc/test/rtp_file_writer.cc
index d9e058646814a0dc80b75b8af0159521f20ab167..59a5a1c060d7fb6b13379f699b794dfc8eeac16b 100644
--- a/webrtc/test/rtp_file_writer.cc
+++ b/webrtc/test/rtp_file_writer.cc
@@ -28,13 +28,13 @@ static const char kFirstLine[] = "#!rtpplay1.0 0.0.0.0/0\n";
 class RtpDumpWriter : public RtpFileWriter {
  public:
   explicit RtpDumpWriter(FILE* file) : file_(file) {
-    RTC_CHECK(file_ != NULL);
+    RTC_CHECK(file_ != nullptr);
     Init();
   }
   virtual ~RtpDumpWriter() {
-    if (file_ != NULL) {
+    if (file_ != nullptr) {
       fclose(file_);
-      file_ = NULL;
+      file_ = nullptr;
     }
   }
 
@@ -92,16 +92,16 @@ class RtpDumpWriter : public RtpFileWriter {
 RtpFileWriter* RtpFileWriter::Create(FileFormat format,
                                      const std::string& filename) {
   FILE* file = fopen(filename.c_str(), "wb");
-  if (file == NULL) {
+  if (file == nullptr) {
     printf("ERROR: Can't open file: %s\n", filename.c_str());
-    return NULL;
+    return nullptr;
   }
   switch (format) {
     case kRtpDump:
       return new RtpDumpWriter(file);
   }
   fclose(file);
-  return NULL;
+  return nullptr;
 }
 
 }  // namespace test