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

mojo/services/dbus_echo/echo.mojom

Index: mojo/services/dbus_echo/echo.mojom
diff --git a/tools/clang/blink_gc_plugin/tests/cycle_sub.cpp b/mojo/services/dbus_echo/echo.mojom
similarity index 53%
copy from tools/clang/blink_gc_plugin/tests/cycle_sub.cpp
copy to mojo/services/dbus_echo/echo.mojom
index 5c0d9e6768bed5b8a2d1a6855d9446208aa695ed..c2ad42a7f7e7ee195679c50daf49b100bb62ddd1 100644
--- a/tools/clang/blink_gc_plugin/tests/cycle_sub.cpp
+++ b/mojo/services/dbus_echo/echo.mojom
@@ -2,13 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "cycle_sub.h"
+module mojo {
 
-namespace WebCore {
+[Peer=EchoClient]
+interface EchoService {
+  Echo(string to_echo) => (string echoed);
+};
 
-void B::trace(Visitor* visitor) {
-    visitor->trace(m_c);
-    A::trace(visitor);
-}
+[Peer=EchoService]
+interface EchoClient {
+};
 
 }