Replace NULL with nullptr in all C++ files.

webrtc/test/testsupport/frame_writer.cc

Index: webrtc/test/testsupport/frame_writer.cc
diff --git a/webrtc/test/testsupport/frame_writer.cc b/webrtc/test/testsupport/frame_writer.cc
index 1b9e8a82efda2f1dd41e78a7ed0a8059e5847a3d..4423c440e049ad2272f4fb8ab11976758ee2e2f8 100644
--- a/webrtc/test/testsupport/frame_writer.cc
+++ b/webrtc/test/testsupport/frame_writer.cc
@@ -19,8 +19,7 @@ FrameWriterImpl::FrameWriterImpl(std::string output_filename,
                                  size_t frame_length_in_bytes)
     : output_filename_(output_filename),
       frame_length_in_bytes_(frame_length_in_bytes),
-      output_file_(NULL) {
-}
+      output_file_(nullptr) {}
 
 FrameWriterImpl::~FrameWriterImpl() {
   Close();
@@ -33,7 +32,7 @@ bool FrameWriterImpl::Init() {
     return false;
   }
   output_file_ = fopen(output_filename_.c_str(), "wb");
-  if (output_file_ == NULL) {
+  if (output_file_ == nullptr) {
     fprintf(stderr, "Couldn't open output file for writing: %s\n",
             output_filename_.c_str());
     return false;
@@ -42,9 +41,9 @@ bool FrameWriterImpl::Init() {
 }
 
 void FrameWriterImpl::Close() {
-  if (output_file_ != NULL) {
+  if (output_file_ != nullptr) {
     fclose(output_file_);
-    output_file_ = NULL;
+    output_file_ = nullptr;
   }
 }
 
@@ -52,8 +51,8 @@ size_t FrameWriterImpl::FrameLength() { return frame_length_in_bytes_; }
 
 bool FrameWriterImpl::WriteFrame(uint8_t* frame_buffer) {
   assert(frame_buffer);
-  if (output_file_ == NULL) {
-    fprintf(stderr, "FrameWriter is not initialized (output file is NULL)\n");
+  if (output_file_ == nullptr) {
+    fprintf(stderr, "FrameWriter is not initialized (output file is null)\n");
     return false;
   }
   size_t bytes_written = fwrite(frame_buffer, 1, frame_length_in_bytes_,