[mojo] Implement pipe fusion API

mojo/public/c/system/message_pipe.h

Index: mojo/public/c/system/message_pipe.h
diff --git a/mojo/public/c/system/message_pipe.h b/mojo/public/c/system/message_pipe.h
index 10da5256740546f076dd42d29b871b4a2924e1d7..01435f1b09d6232002f03f464440628a443d4c04 100644
--- a/mojo/public/c/system/message_pipe.h
+++ b/mojo/public/c/system/message_pipe.h
@@ -181,6 +181,32 @@ MOJO_SYSTEM_EXPORT MojoResult
                     uint32_t* num_handles,  // Optional in/out.
                     MojoReadMessageFlags flags);
 
+// Fuses two message pipe endpoints together. Given two pipes:
+//
+//     A <-> B    and    C <-> D
+//
+// Fusing handle B and handle C results in a single pipe:
+//
+//     A <-> D
+//
+// Handles B and C are closed. Any unread messages at C will eventually be
+// delivered to A, and any unread messages at B will eventually be delivered to
+// D.
+//
+// NOTE: A handle may only be fused as long as it is open and has not been
+// written to.
+//
+// Returns:
+//   |MOJO_RESULT_OK| on success.
+//   |MOJO_RESULT_FAILED_PRECONDITION| if both handles were valid message pipe
+//       handles but could not be merged (e.g. one of them has been written to).
+//       In this case the pipes are not fused and both handles are closed.
+//   |MOJO_INVALID_ARGUMENT| if either handle is not a fusable message pipe
+//       handle. In this case both handles remain unaffected.

[yzshen1, 2016/03/10 23:49:03]

Is it easier if we always close the handles? That is one less thing to consider
for the users, and invalid argument is pretty rare. WDYT?

[Ken Rockot(use gerrit already), 2016/03/11 00:32:46]

SGTM - I considered both and didn't feel strongly about it, but it makes the C++
APIs much cleaner IMHO. So done.

+MOJO_SYSTEM_EXPORT MojoResult
+    MojoFuseMessagePipes(MojoHandle handle0, MojoHandle handle1);
+
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif