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

BUG=None
TEST=None
R=sbc@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_nacl_sdk;master.tryserver.chromium.mac:mac_nacl_sdk;master.tryserver.chromium.win:win_nacl_sdk
BUG=646185

Review-Url: https://codereview.chromium.org/2342353003
Cr-Commit-Position: refs/heads/master@{#419332}

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

@@ -8,6 +8,10 @@
 
 #include "ppapi/cpp/var_dictionary.h"
 
+#ifdef WIN32
+#  undef PostMessage
+#endif
+
 VpnProviderHelper::VpnProviderHelper(pp::Instance* instance)
     : instance_(instance),
       thread_(pp::InstanceHandle(instance)),