Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_mixer/audio_mixer_impl.cc

Index: webrtc/modules/audio_mixer/audio_mixer_impl.cc
diff --git a/webrtc/modules/audio_mixer/audio_mixer_impl.cc b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
index 3da1be227071da85ea62b08f4ec3cdf3611286e5..e7c63d8778a19ee1622c7e5b52d657b54f539920 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_impl.cc
+++ b/webrtc/modules/audio_mixer/audio_mixer_impl.cc
@@ -220,8 +220,8 @@ void AudioMixerImpl::Mix(size_t number_of_channels,
     }
 
     audio_frame_for_mixing->UpdateFrame(
-        -1, time_stamp_, NULL, 0, OutputFrequency(), AudioFrame::kNormalSpeech,
-        AudioFrame::kVadPassive, number_of_channels);
+        -1, time_stamp_, nullptr, 0, OutputFrequency(),
+        AudioFrame::kNormalSpeech, AudioFrame::kVadPassive, number_of_channels);
 
     time_stamp_ += static_cast<uint32_t>(sample_size_);