Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_mixer/audio_mixer_impl.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 202 matching lines @@
   AudioFrameList mix_list;
   {
     rtc::CritScope lock(&crit_);
     mix_list = GetAudioFromSources();
 
     for (const auto& frame : mix_list) {
       RemixFrame(number_of_channels, frame);
     }
 
     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_);
 
     use_limiter_ = mix_list.size() > 1;
 
     // We only use the limiter if we're actually mixing multiple streams.
     MixFromList(audio_frame_for_mixing, mix_list, use_limiter_);
   }
 
   if (audio_frame_for_mixing->samples_per_channel_ == 0) {
@@ skipping 133 matching lines @@
 
   const auto iter = FindSourceInList(audio_source, &audio_source_list_);
   if (iter != audio_source_list_.end()) {
     return (*iter)->is_mixed;
   }
 
   LOG(LS_ERROR) << "Audio source unknown";
   return false;
 }
 }  // namespace webrtc