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

mojo/system/proxy_message_pipe_endpoint.cc

Index: mojo/system/proxy_message_pipe_endpoint.cc
diff --git a/mojo/system/proxy_message_pipe_endpoint.cc b/mojo/system/proxy_message_pipe_endpoint.cc
index a52099f8d0b9d40c1f8b10029ecc98d942fb522a..fda3f42c189b5744cebd4d855598bdbedf487ab9 100644
--- a/mojo/system/proxy_message_pipe_endpoint.cc
+++ b/mojo/system/proxy_message_pipe_endpoint.cc
@@ -108,7 +108,7 @@ void ProxyMessagePipeEndpoint::Detach() {
   DCHECK(is_attached());
 
   channel_endpoint_->DetachFromMessagePipe();
-  channel_endpoint_ = NULL;
+  channel_endpoint_ = nullptr;
   is_running_ = false;
   paused_message_queue_.Clear();
 }