Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc

Index: webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc
diff --git a/webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc b/webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc
index 3835e935d57d8718c59ae3b24003d48fac2b5372..9f9eeda81d1704729eeaf381b6c10b83b4c34a81 100644
--- a/webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc
+++ b/webrtc/modules/audio_coding/codecs/cng/cng_unittest.cc
@@ -48,11 +48,11 @@ void CngTest::SetUp() {
   const std::string file_name =
         webrtc::test::ResourcePath("audio_coding/testfile32kHz", "pcm");
   input_file = fopen(file_name.c_str(), "rb");
-  ASSERT_TRUE(input_file != NULL);
+  ASSERT_TRUE(input_file != nullptr);
   ASSERT_EQ(640, static_cast<int32_t>(fread(speech_data_, sizeof(int16_t),
                                              640, input_file)));
   fclose(input_file);
-  input_file = NULL;
+  input_file = nullptr;
 }
 
 void CngTest::TestCngEncode(int sample_rate_hz, int quality) {