Mojo: Add static factory functions to MessagePipe.

mojo/system/message_pipe.h

Index: mojo/system/message_pipe.h
diff --git a/mojo/system/message_pipe.h b/mojo/system/message_pipe.h
index f39c7b04b334272a074c8dda857e68e67085c828..626d031ceb3517d8e7e815342b42e0913114e417 100644
--- a/mojo/system/message_pipe.h
+++ b/mojo/system/message_pipe.h
@@ -37,9 +37,22 @@ class MOJO_SYSTEM_IMPL_EXPORT MessagePipe
   MessagePipe(scoped_ptr<MessagePipeEndpoint> endpoint0,
               scoped_ptr<MessagePipeEndpoint> endpoint1);
 
-  // Convenience constructor that constructs a |MessagePipe| with two new
-  // |LocalMessagePipeEndpoint|s.
-  MessagePipe();
+  // Constructs a |MessagePipe| with two new |LocalMessagePipeEndpoint|s.
+  struct LocalLocal {};
+  explicit MessagePipe(LocalLocal);
+
+  // Constructs a |MessagePipe| with a |LocalMessagePipeEndpoint| on port 0 and
+  // a |ProxyMessagePipeEndpoint| on port 1.
+  struct LocalProxy {};
+  explicit MessagePipe(LocalProxy);
+
+  // Constructs a |MessagePipe| with a |ProxyMessagePipeEndpoint| on port 0 and
+  // a |LocalMessagePipeEndpoint| on port 1.
+  // Note: This is really only needed in tests (outside of tests, this
+  // configuration arises from a local message pipe having its port 0
+  // "converted" using |ConvertLocalToProxy()|).
+  struct ProxyLocal {};
+  explicit MessagePipe(ProxyLocal);
 
   // Gets the other port number (i.e., 0 -> 1, 1 -> 0).
   static unsigned GetPeerPort(unsigned port);