Mojo: NULL -> nullptr in mojo/system and mojo/embedder.

mojo/system/message_pipe.cc

Index: mojo/system/message_pipe.cc
diff --git a/mojo/system/message_pipe.cc b/mojo/system/message_pipe.cc
index 4a75a297bad0d936c28c732f9341d00c77945ed6..e2aabe37c176ee047610daa1305ac08f0fc9340a 100644
--- a/mojo/system/message_pipe.cc
+++ b/mojo/system/message_pipe.cc
@@ -184,7 +184,7 @@ scoped_refptr<ChannelEndpoint> MessagePipe::ConvertLocalToProxy(unsigned port) {
 
 MojoResult MessagePipe::EnqueueMessage(unsigned port,
                                        scoped_ptr<MessageInTransit> message) {
-  return EnqueueMessageInternal(port, message.Pass(), NULL);
+  return EnqueueMessageInternal(port, message.Pass(), nullptr);
 }
 
 bool MessagePipe::Attach(unsigned port, ChannelEndpoint* channel_endpoint) {