[mojo] Implement pipe fusion API

mojo/edk/system/message_pipe_dispatcher.h

Index: mojo/edk/system/message_pipe_dispatcher.h
diff --git a/mojo/edk/system/message_pipe_dispatcher.h b/mojo/edk/system/message_pipe_dispatcher.h
index 7400515f93036670ec130d3fbf61b20cd31d0fb6..b457ab99f633d23e219dbcece6d59fc49cf3fbff 100644
--- a/mojo/edk/system/message_pipe_dispatcher.h
+++ b/mojo/edk/system/message_pipe_dispatcher.h
@@ -39,6 +39,11 @@ class MessagePipeDispatcher : public Dispatcher {
                         uint64_t pipe_id,
                         int endpoint);
 
+  // Fuses this pipe with |other|. Returns |true| on success or |false| on
+  // failure. Regardless of the return value, both dispatchers are closed by
+  // this call.
+  bool Fuse(MessagePipeDispatcher* other);
+
   // Dispatcher:
   Type GetType() const override;
   MojoResult Close() override;