Fix ppapi sample when built on windows.

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

Index: native_client_sdk/src/examples/api/vpn_provider/vpn_provider.cc
diff --git a/native_client_sdk/src/examples/api/vpn_provider/vpn_provider.cc b/native_client_sdk/src/examples/api/vpn_provider/vpn_provider.cc
index 88e5aaa3d70b6df1530aec8baf06e7b6423099ce..416535d79d0430dd26ca54ec86af9b95b2cb5c91 100644
--- a/native_client_sdk/src/examples/api/vpn_provider/vpn_provider.cc
+++ b/native_client_sdk/src/examples/api/vpn_provider/vpn_provider.cc
@@ -13,6 +13,10 @@
 
 #include "vpn_provider_helper.h"
 
+#ifdef WIN32
+# undef PostMessage
+#endif
+
 class VpnProviderInstance : public pp::Instance {
  public:
   explicit VpnProviderInstance(PP_Instance instance)