Add multiplexing of message pipes in the new EDK.

mojo/edk/system/message_in_transit.cc

Index: mojo/edk/system/message_in_transit.cc
diff --git a/mojo/edk/system/message_in_transit.cc b/mojo/edk/system/message_in_transit.cc
index b85efc7fe583c92083e618ea9f0eede1015854ad..05fd98f08c8705f57944c5989af701d5f4696dbb 100644
--- a/mojo/edk/system/message_in_transit.cc
+++ b/mojo/edk/system/message_in_transit.cc
@@ -166,6 +166,7 @@ void MessageInTransit::ConstructorHelper(Type type,
   header()->type = type;
   header()->num_bytes = num_bytes;
   header()->unused = 0;
+  header()->route_id = 0;
   // Note: If dispatchers are subsequently attached, then |total_size| will have
   // to be adjusted.
   UpdateTotalSize();