Mojo: Change the way message pipes are passed over channels.

mojo/edk/system/message_in_transit.h

Index: mojo/edk/system/message_in_transit.h
diff --git a/mojo/edk/system/message_in_transit.h b/mojo/edk/system/message_in_transit.h
index 06b4f6ce4776386d080c704613939342af0c4546..2b9da6d0822bf7bdc4ac830c18077e568efb874b 100644
--- a/mojo/edk/system/message_in_transit.h
+++ b/mojo/edk/system/message_in_transit.h
@@ -59,9 +59,10 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageInTransit {
   // Subtypes for type |kTypeMessagePipe|:
   // Nothing currently.
   // Subtypes for type |kTypeChannel|:
-  static const Subtype kSubtypeChannelRunMessagePipeEndpoint = 0;
+  static const Subtype kSubtypeChannelAttachAndRunEndpoint = 0;
   static const Subtype kSubtypeChannelRemoveMessagePipeEndpoint = 1;
   static const Subtype kSubtypeChannelRemoveMessagePipeEndpointAck = 2;
+
   // Subtypes for type |kTypeRawChannel|:
   static const Subtype kSubtypeRawChannelPosixExtraPlatformHandles = 0;