Expand site isolation postMessage testing

https://bugs.webkit.org/show_bug.cgi?id=262590
rdar://116436305

Reviewed by Pascoe.

RemoteDOMWindow::postMessage has some branches that weren't tested.

* LayoutTests/http/tests/site-isolation/post-message-expected.txt:
* LayoutTests/http/tests/site-isolation/post-message.html:

Canonical link: https://commits.webkit.org/268827@main

LayoutTests/http/tests/site-isolation/post-message-expected.txt

@@ -1,5 +1,8 @@
+PASS posting a message to about:blank threw an exception: SyntaxError: The string did not match the expected pattern.
+PASS posting a non-serializable message threw an exception: DataCloneError: The object can not be cloned.
 PASS successfullyParsed is true
 
 TEST COMPLETE
 PASS received 'iframe received hello'
+PASS received 'iframe received world'
 

LayoutTests/http/tests/site-isolation/post-message.html

@@ -6,12 +6,30 @@
     testRunner.waitUntilDone();
 }
 
+let messageCount = 0;
 addEventListener("message", (event) => {
     testPassed("received '" + event.data + "'");
-    testRunner.notifyDone();
+    messageCount = messageCount + 1;
+    if (messageCount == 2) { testRunner.notifyDone() }
 });
 
-onload = ()=>{ frame.contentWindow.postMessage("hello", "*") }
+onload = ()=>{
+    frame.contentWindow.postMessage("hello", "*")
+    frame.contentWindow.postMessage("should not be received", "http://webkit.org")
+    frame.contentWindow.postMessage("world", "http://localhost:8000")
+
+    try {
+        frame.contentWindow.postMessage("should throw", "about:blank")
+    } catch (e) {
+        testPassed("posting a message to about:blank threw an exception: " + e)
+    }
+
+    try {
+        frame.contentWindow.postMessage(window.performance, "/")
+    } catch (e) {
+        testPassed("posting a non-serializable message threw an exception: " + e)
+    }
+}
 
 </script>
 <iframe src="http://localhost:8000/site-isolation/resources/post-message-to-parent.html" id="frame"></iframe>