Require an explicit ServiceName annotation for interfaces in C++.

services/js/test/echo_service.mojom

 // 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;
 
+[ServiceName="mojo::EchoService"]
 interface EchoService {
   EchoString(string? value) => (string? value);
 
   // Provide the EchoService to the echo_target.js
   // application. The full URL for echo_target.js is
   // computed relative to this application's URL.
   // Return after echo_target has called EchoString().
   ShareEchoService() => (bool ok);
 
   // Make it stop.
   Quit();
 };