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

mojo/services/files/interfaces/files.mojom

 // Copyright 2015 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.
 
 [DartPackage="mojo_services"]
 module mojo.files;
 
 import "files/interfaces/directory.mojom";
 import "files/interfaces/types.mojom";
 
+[ServiceName="mojo::files::Files"]
 interface Files {
   // Opens the root directory for the file system with the given name; null
   // yields the default file system, if any.
   OpenFileSystem(string? file_system, Directory& directory) => (Error error);
 };