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

examples/surfaces_app/child.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.
 
 import "mojo/services/geometry/interfaces/geometry.mojom";
 import "mojo/services/surfaces/interfaces/quads.mojom";
 import "mojo/services/surfaces/interfaces/surface_id.mojom";
 
+[ServiceName="Child"]
 interface Child {
   ProduceFrame(mojo.Color color, mojo.Size size) =>
       (mojo.SurfaceId id);
 };