Replace NULL with nullptr in all C++ files.

webrtc/voice_engine/transmit_mixer_unittest.cc

Index: webrtc/voice_engine/transmit_mixer_unittest.cc
diff --git a/webrtc/voice_engine/transmit_mixer_unittest.cc b/webrtc/voice_engine/transmit_mixer_unittest.cc
index bd390ae1d5a41f477feda79b13e34cfed42688e8..0d30f28b5f4bc739baea20118c558ca592dab6c6 100644
--- a/webrtc/voice_engine/transmit_mixer_unittest.cc
+++ b/webrtc/voice_engine/transmit_mixer_unittest.cc
@@ -28,11 +28,11 @@ class MediaCallback : public VoEMediaProcess {
 // TODO(andrew): Mock VoEMediaProcess, and verify the behavior when calling
 // PrepareDemux().
 TEST(TransmitMixerTest, RegisterExternalMediaCallback) {
-  TransmitMixer* tm = NULL;
+  TransmitMixer* tm = nullptr;
   ASSERT_EQ(0, TransmitMixer::Create(tm, 0));
-  ASSERT_TRUE(tm != NULL);
+  ASSERT_TRUE(tm != nullptr);
   MediaCallback callback;
-  EXPECT_EQ(-1, tm->RegisterExternalMediaProcessing(NULL,
+  EXPECT_EQ(-1, tm->RegisterExternalMediaProcessing(nullptr,
                                                     kRecordingPreprocessing));
   EXPECT_EQ(-1, tm->RegisterExternalMediaProcessing(&callback,
                                                     kPlaybackPerChannel));