Replace NULL with nullptr in all C++ files.

webrtc/voice_engine/voe_base_impl.cc

Index: webrtc/voice_engine/voe_base_impl.cc
diff --git a/webrtc/voice_engine/voe_base_impl.cc b/webrtc/voice_engine/voe_base_impl.cc
index 6432e502ad0fec6501974e6a2b4a0c0b173789d1..c885e96d60af6f7fe2b7f68a85aa18ffe0130834 100644
--- a/webrtc/voice_engine/voe_base_impl.cc
+++ b/webrtc/voice_engine/voe_base_impl.cc
@@ -755,7 +755,7 @@ int VoEBaseImpl::AssociateSendChannel(int channel,
 
   voe::ChannelOwner ch = shared_->channel_manager().GetChannel(channel);
   voe::Channel* channel_ptr = ch.channel();
-  if (channel_ptr == NULL) {
+  if (channel_ptr == nullptr) {
     shared_->SetLastError(VE_CHANNEL_NOT_VALID, kTraceError,
         "AssociateSendChannel() failed to locate channel");
     return -1;
@@ -763,7 +763,7 @@ int VoEBaseImpl::AssociateSendChannel(int channel,
 
   ch = shared_->channel_manager().GetChannel(accociate_send_channel);
   voe::Channel* accociate_send_channel_ptr = ch.channel();
-  if (accociate_send_channel_ptr == NULL) {
+  if (accociate_send_channel_ptr == nullptr) {
     shared_->SetLastError(VE_CHANNEL_NOT_VALID, kTraceError,
         "AssociateSendChannel() failed to locate accociate_send_channel");
     return -1;