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

mojo/system/message_in_transit.h

Index: mojo/system/message_in_transit.h
diff --git a/mojo/system/message_in_transit.h b/mojo/system/message_in_transit.h
index c812d56c66b3d5847e272f2ece95ff52b7428e15..9101f365a8cab4ed38e1c2de76d798189ffc8f21 100644
--- a/mojo/system/message_in_transit.h
+++ b/mojo/system/message_in_transit.h
@@ -106,7 +106,7 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageInTransit {
     const void* transport_data_buffer() const {
       return (total_size() > main_buffer_size())
                  ? static_cast<const char*>(buffer_) + main_buffer_size()
-                 : NULL;
+                 : nullptr;
     }
     size_t transport_data_buffer_size() const {
       return total_size() - main_buffer_size();