Replace NULL with nullptr in all C++ files.

webrtc/voice_engine/voice_engine_impl.cc

Index: webrtc/voice_engine/voice_engine_impl.cc
diff --git a/webrtc/voice_engine/voice_engine_impl.cc b/webrtc/voice_engine/voice_engine_impl.cc
index 5b92d27313bad3546e6d48c99520031e56325dcf..16a0684db0b3617e2a7a69d7e774a26f1558f13d 100644
--- a/webrtc/voice_engine/voice_engine_impl.cc
+++ b/webrtc/voice_engine/voice_engine_impl.cc
@@ -31,7 +31,7 @@ static int32_t gVoiceEngineInstanceCounter = 0;
 
 VoiceEngine* GetVoiceEngine() {
   VoiceEngineImpl* self = new VoiceEngineImpl();
-  if (self != NULL) {
+  if (self != nullptr) {
     self->AddRef();  // First reference.  Released in VoiceEngine::Delete.
     gVoiceEngineInstanceCounter++;
   }
@@ -110,13 +110,13 @@ int VoiceEngine::SetTraceCallback(TraceCallback* callback) {
 }
 
 bool VoiceEngine::Delete(VoiceEngine*& voiceEngine) {
-  if (voiceEngine == NULL)
+  if (voiceEngine == nullptr)
     return false;
 
   VoiceEngineImpl* s = static_cast<VoiceEngineImpl*>(voiceEngine);
   // Release the reference that was added in GetVoiceEngine.
   int ref = s->Release();
-  voiceEngine = NULL;
+  voiceEngine = nullptr;
 
   if (ref != 0) {
     WEBRTC_TRACE(