Mark PostMessageOptions argument as optional on MessagePort

Follow-up to e3f1cc8. Fixes #4137.

Author: dtapuska

source

@@ -96466,8 +96466,8 @@ interface <dfn>MessageChannel</dfn> {
 
   <pre><code class="idl" data-x="">[Exposed=(Window,Worker,AudioWorklet), <span>Transferable</span>]
 interface <dfn>MessagePort</dfn> : <span>EventTarget</span> {
-  void <span data-x="dom-MessagePort-postMessage">postMessage</span>(any message, optional sequence&lt;<span data-x="idl-object">object</span>&gt; transfer = []);
-  void <span data-x="dom-MessagePort-postMessage-options">postMessage</span>(any message, <span>PostMessageOptions</span> options);
+  void <span data-x="dom-MessagePort-postMessage">postMessage</span>(any message, sequence&lt;<span data-x="idl-object">object</span>&gt; transfer);
+  void <span data-x="dom-MessagePort-postMessage-options">postMessage</span>(any message, optional <span>PostMessageOptions</span> options);
   void <span data-x="dom-MessagePort-start">start</span>();
   void <span data-x="dom-MessagePort-close">close</span>();