Mojom: Blanket change to mark all pointer/handle fields as nullable.

mojo/public/interfaces/application/shell.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/public/interfaces/application/service_provider.mojom"
 
 module mojo {
 
 // The Shell is the finder and launcher of Applications. An Application uses
 // it's Shell interface to connect to other Applications.
 [Client=Application]
 interface Shell {
   // Loads url. mojo:{service} will result in the user of the value of the
   // --origin flag to the shell being used.
-  ConnectToApplication(string application_url, ServiceProvider& provider);
+  ConnectToApplication(string? application_url, ServiceProvider&? provider);
 };
 
 }