Add support for threadsafe completion callback factory.

ppapi/cpp/websocket.h

Index: ppapi/cpp/websocket.h
diff --git a/ppapi/cpp/websocket.h b/ppapi/cpp/websocket.h
index a34fdb7731ebfad36af041788f3a2e2c389340f5..0c72dc0eda9fd578a9bd8d714e7a72e94a1902a9 100644
--- a/ppapi/cpp/websocket.h
+++ b/ppapi/cpp/websocket.h
@@ -12,6 +12,11 @@
 /// This file defines the WebSocket interface providing bi-directional,
 /// full-duplex, communications over a single TCP socket.
 
+// Windows headers will redefine SendMessage.
+#ifdef SendMessage
+#undef SendMessage
+#endif
+
 namespace pp {
 
 class CompletionCallback;