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

examples/indirect_service/indirect_service_demo.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.examples;
 
+[ServiceName="mojo::examples::IntegerService"]
 interface IntegerService {
   Increment() => (int32 value);
 };
 
+[ServiceName="mojo::examples::IndirectIntegerService"]
 interface IndirectIntegerService {
   Set(IntegerService? service);
   Get(IntegerService&? service);
 };