Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc

Index: webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc
diff --git a/webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc b/webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc
index 8748162eb1b88745d45d30022e522e3d21dcf439..5be3efe5453cf4c7f8919131515110f83a1dd450 100644
--- a/webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc
+++ b/webrtc/modules/audio_coding/codecs/opus/opus_fec_test.cc
@@ -75,7 +75,7 @@ void OpusFecTest::SetUp() {
   in_filename_ = test::ResourcePath(get<2>(GetParam()), get<3>(GetParam()));
 
   FILE* fp = fopen(in_filename_.c_str(), "rb");
-  ASSERT_FALSE(fp == NULL);
+  ASSERT_FALSE(fp == nullptr);
 
   // Obtain file size.
   fseek(fp, 0, SEEK_END);
@@ -129,9 +129,8 @@ OpusFecTest::OpusFecTest()
       data_pointer_(0),
       max_bytes_(0),
       encoded_bytes_(0),
-      opus_encoder_(NULL),
-      opus_decoder_(NULL) {
-}
+      opus_encoder_(nullptr),
+      opus_decoder_(nullptr) {}
 
 void OpusFecTest::EncodeABlock() {
   int value = WebRtcOpus_Encode(opus_encoder_,