[nacl-sdk] Fix nacl sdk example on windows.

native_client_sdk/src/examples/api/vpn_provider/vpn_provider_helper.cc

 // 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.
 
 #include "vpn_provider_helper.h"
 
 #include <sstream>
 
 #include "ppapi/cpp/var_dictionary.h"
 
+#ifdef WIN32
+#  undef PostMessage
+#endif
+
 VpnProviderHelper::VpnProviderHelper(pp::Instance* instance)
     : instance_(instance),
       thread_(pp::InstanceHandle(instance)),
       vpn_(pp::InstanceHandle(instance)),
       factory_(this),
       send_packet_pending_(false) {
   thread_.Start();
 }
 
 void VpnProviderHelper::Init() {
@@ skipping 98 matching lines @@
     instance_->PostMessage(ss.str());
 
     vpn_.SendPacket(
         send_packet_queue_.front(),
         factory_.NewCallback(&VpnProviderHelper::SendPacketCompletionCallback));
     send_packet_queue_.pop();
   } else {
     send_packet_pending_ = false;
   }
 }