Move //mojo/shell to //shell

shell/external_application_registrar.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;
 
 import "mojo/public/interfaces/application/shell.mojom";
 
 // This interface allows applications running outside the auspices of a Shell
 // to request registration at the given application_url.
 // The shell implementing this interface will make the calling application
 // available to other apps at that URL and pass back a Shell bound to
 // an impl capable of servicing the external application's connection requests.
 interface ExternalApplicationRegistrar {
   Register(string application_url) => (Shell shell);
 };