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

mojo/system/channel_endpoint.cc

Index: mojo/system/channel_endpoint.cc
diff --git a/mojo/system/channel_endpoint.cc b/mojo/system/channel_endpoint.cc
index dd0deb2a522bea71be3022b26dceecff1100a66f..d788f90b30563a86ecc23bb434a3abb3c24c4521 100644
--- a/mojo/system/channel_endpoint.cc
+++ b/mojo/system/channel_endpoint.cc
@@ -85,7 +85,7 @@ void ChannelEndpoint::DetachFromChannel() {
   DCHECK_NE(local_id_, MessageInTransit::kInvalidEndpointId);
   // TODO(vtl): Once we combine "run" into "attach", |remote_id_| should valid
   // here as well.
-  channel_ = NULL;
+  channel_ = nullptr;
   local_id_ = MessageInTransit::kInvalidEndpointId;
   remote_id_ = MessageInTransit::kInvalidEndpointId;
 }