Revert "relax postMessage type requirements"

This reverts commit fb56d2f.

src/Webapi/Dom/Webapi__Dom__Window.re

@@ -95,9 +95,9 @@ module Impl = (T: {
     (~url: string, ~name: string, ~features: string=?) => option(Dom.window) =
     "open"; /* yes, features is a stringly typed list of key value pairs, sigh */
   [@mel.send.pipe: t_window]
-  external postMessage: ('a) => unit = "postMessage"; /* experimental-ish?, Web Messaging */
+  external postMessage: ('a, string) => unit = "postMessage"; /* experimental-ish?, Web Messaging */
   [@mel.send.pipe: t_window]
-  external postMessageWithTransfers: ('a, array(transferable)) => unit =
+  external postMessageWithTransfers: ('a, string, array(transferable)) => unit =
     "postMessage"; /* experimental-ish?, Web Messaging */
   [@mel.send.pipe: t_window] external print: unit = "print";
   [@mel.send.pipe: t_window] external prompt: string => string = "prompt";