Separate mojo/shell into a runner and the application manager (shell)

mojo/shell/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 ChildController {
-  // 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);
-};