More informative error messages for non-Transferables.

LayoutTests/fast/dom/Window/window-postmessage-args.html

Index: LayoutTests/fast/dom/Window/window-postmessage-args.html
diff --git a/LayoutTests/fast/dom/Window/window-postmessage-args.html b/LayoutTests/fast/dom/Window/window-postmessage-args.html
index f46acfee8ef46fe5fd3237b02125d383938dd37d..18277f614c97899686e2e025e94c2639633d4d34 100644
--- a/LayoutTests/fast/dom/Window/window-postmessage-args.html
+++ b/LayoutTests/fast/dom/Window/window-postmessage-args.html
@@ -42,8 +42,9 @@ function tryPostMessage(first, second, third, shouldFail) {
 document.getElementById("description").innerHTML = "Test that the second argument of window.postMessage is ignored or triggers an error if it is not a message port. You should see PASS message '1' through '7', followed by 'done', with messages 4-7 received below.<br><br>";
 
 tryPostMessage('1', '*', 1, true);
-tryPostMessage('2', '*', 'c', true);
+tryPostMessage('2', '*', 'c', true); // Legacy overload resolution will consider 3rd argument to be the (string) origin.
 tryPostMessage('3', '*', { x: 1 }, true);
+tryPostMessage('3a', { x: 1 }, '*', true); // Legacy argument order.
 tryPostMessage('4', '*', window);  // Passes because window has a "length" attribute of value '0', so it looks like an array
 tryPostMessage('4a', window, '*'); // Legacy argument order.
 tryPostMessage('5', '*', null);