Implement a trivial Mojo EchoService that can be connected to over DBus

mojo/shell/external_service.mojom

Index: mojo/shell/external_service.mojom
diff --git a/mojo/shell/external_service.mojom b/mojo/shell/external_service.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..f76f5feb9b766676d63794017ef7180215fa6ef1
--- /dev/null
+++ b/mojo/shell/external_service.mojom
@@ -0,0 +1,16 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module mojo {
+
+[Peer=ExternalServiceHost]
+interface ExternalService {
+  Activate(handle<message_pipe> shell_handle);
+};
+
+[Peer=ExternalService]
+interface ExternalServiceHost {
+};
+
+}