Make ChannelHandle::socket optional in mus' ChannelHandle mojom.

services/ui/public/interfaces/channel_handle.mojom

 // Copyright 2016 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 ui.mojom;
 
 // See IPC::ChannelHandle in src/ipc/ipc_channel_handle.h
 struct ChannelHandle {
   string name;
 
   // On POSIX, it is one endpoint of a socket pair. On Windows, it is not used.
-  handle socket;
+  handle? socket;
 
   // Message pipe handle used for ChannelMojo.
   handle<message_pipe>? mojo_handle;
 };