Add an XPC implementation of sandbox::MessageServer.

sandbox/mac/xpc_stubs_header.fragment

 // 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.
 
 #ifndef SANDBOX_MAC_XPC_STUBS_HEADER_FRAGMENT_
 #define SANDBOX_MAC_XPC_STUBS_HEADER_FRAGMENT_
 
+#include <bsm/libbsm.h>
+
+#include "sandbox/sandbox_export.h"
+
 // Declare or include public types.
 #if !defined(MAC_OS_X_VERSION_10_7) || \
     MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
 
 extern "C" {
 typedef void* xpc_object_t;
 }  // extern "C"
 
 #else
 
 #include <xpc/xpc.h>
 
 #endif
 
 // Declare private types.
 extern "C" {
 typedef struct _xpc_pipe_s* xpc_pipe_t;
 }  // extern "C"
 
 #endif  // SANDBOX_MAC_XPC_STUBS_HEADER_FRAGMENT_