Fix the //sandbox/mac build when using a modern (10.7+) SDK.

sandbox/mac/xpc.h

 // 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.
 
 // This file provides forward declarations for XPC symbols that are not
 // present in the 10.6 SDK. It uses generate_stubs to produce code to
 // dynamically load the libxpc.dylib library and set up a stub table, with
 // the same names as the real XPC functions.
 
 #ifndef SANDBOX_MAC_XPC_H_
 #define SANDBOX_MAC_XPC_H_
 
+#include <AvailabilityMacros.h>
 #include <mach/mach.h>
 
 #include "sandbox/sandbox_export.h"
 
-// C++ library loader.
-#include "sandbox/mac/xpc_stubs.h"
-
 // Declares XPC object types. This includes <xpc/xpc.h> if available.
 #include "sandbox/mac/xpc_stubs_header.fragment"
 
 #if !defined(MAC_OS_X_VERSION_10_7) || \
     MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
 
+// C++ library loader.
+#include "sandbox/mac/xpc_stubs.h"
+
 extern "C" {
 // Signatures for XPC public functions that are loaded by xpc_stubs.h.
 #include "sandbox/mac/xpc_stubs.sig"
 // Signatures for private XPC functions.
 #include "sandbox/mac/xpc_private_stubs.sig"
 }  // extern "C"
 
 #else
 
 // Signatures for private XPC functions.
 extern "C" {
 #include "sandbox/mac/xpc_private_stubs.sig"
 }  // extern "C"
 
 #endif
 
 namespace sandbox {
 
 // Dynamically loads the XPC library.
 bool SANDBOX_EXPORT InitializeXPC();
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_MAC_XPC_H_