Replace NULL with nullptr in all C++ files.

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

Index: webrtc/modules/audio_coding/codecs/opus/opus_speed_test.cc
diff --git a/webrtc/modules/audio_coding/codecs/opus/opus_speed_test.cc b/webrtc/modules/audio_coding/codecs/opus/opus_speed_test.cc
index 7165d29c8b4d24c6b26df10afe24ef803c3351f3..f536e98d1eec1599b4c4e799e961527fd70f600a 100644
--- a/webrtc/modules/audio_coding/codecs/opus/opus_speed_test.cc
+++ b/webrtc/modules/audio_coding/codecs/opus/opus_speed_test.cc
@@ -35,9 +35,8 @@ OpusSpeedTest::OpusSpeedTest()
     : AudioCodecSpeedTest(kOpusBlockDurationMs,
                           kOpusSamplingKhz,
                           kOpusSamplingKhz),
-      opus_encoder_(NULL),
-      opus_decoder_(NULL) {
-}
+      opus_encoder_(nullptr),
+      opus_decoder_(nullptr) {}
 
 void OpusSpeedTest::SetUp() {
   AudioCodecSpeedTest::SetUp();