LibWeb: Don't discard PostedMessage tasks when closing a worker

The spec expects `postMessage()` to act as if it is invoked
immediately. Since `postMessage()` isn't actually invoked immediately,
keep tasks with source `PostedMessage` in the task queue, so that these
tasks are processed. Fixes a hang when `WorkerGlobalScope.close()` is
called immediately after `postMessage()`.

Tests/LibWeb/Text/input/Worker/worker-close-after-postMessage.js

@@ -0,0 +1,4 @@
+self.onmessage = function () {
+    postMessage("PASS (didn't hang)");
+    self.close();
+};