Shell: Remove the test child stuff.

shell/app_child_process.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.shell;
 
 import "mojo/public/interfaces/application/application.mojom";
 
 interface AppChildController {
+  // Starts the app at the given path (deleting it if |clean_app_path| is true).
   StartApp(string app_path,
            bool clean_app_path,
            mojo.Application& application_request) => (int32 result);
+
+  // Exits the child process now (with no cleanup), with the given exit code.
+  ExitNow(int32 exit_code);
 };