LibWeb: Don't drop messages received before MessagePort is enabled

This change implements following behavior defined in the spec:
https://html.spec.whatwg.org/multipage/web-messaging.html#examples-5
> The start() method, whether called explicitly or implicitly (by
setting onmessage), starts the flow of messages: messages posted on
message ports are initially paused, so that they don't get dropped on
the floor before the script has had a chance to set up its handlers.

Now we don't read bytes from the underlying transport socket until the
message port transitions to the enabled state. This required the
following places to explicitly enable the message port, because now,
when it actually matters, we must do so, or otherwise sent messages will
get stuck:
- `onmessage` attribute setter in DedicatedWorkerGlobalScope, because
  it implicitly sets the onmessage handler for the worker's underlying
  port.
- Stream API operations where the message port enabling steps were
  previously marked as FIXMEs.

Libraries/LibWeb/HTML/DedicatedWorkerGlobalScope.cpp

@@ -66,17 +66,31 @@ WebIDL::ExceptionOr<void> DedicatedWorkerGlobalScope::post_message(JS::Value mes
     return m_internal_port->post_message(message, transfer);
 }
 
-#undef __ENUMERATE
-#define __ENUMERATE(attribute_name, event_name)                                        \
-    void DedicatedWorkerGlobalScope::set_##attribute_name(WebIDL::CallbackType* value) \
-    {                                                                                  \
-        set_event_handler_attribute(event_name, move(value));                          \
-    }                                                                                  \
-    WebIDL::CallbackType* DedicatedWorkerGlobalScope::attribute_name()                 \
-    {                                                                                  \
-        return event_handler_attribute(event_name);                                    \
-    }
-ENUMERATE_DEDICATED_WORKER_GLOBAL_SCOPE_EVENT_HANDLERS(__ENUMERATE)
-#undef __ENUMERATE
+WebIDL::CallbackType* DedicatedWorkerGlobalScope::onmessage()
+{
+    return event_handler_attribute(EventNames::message);
+}
+
+void DedicatedWorkerGlobalScope::set_onmessage(WebIDL::CallbackType* callback)
+{
+    set_event_handler_attribute(EventNames::message, callback);
+
+    // NOTE: This onmessage attribute setter implicitly sets worker's underlying MessagePort's onmessage attribute, so this
+    //       spec behavior also applies here:
+    // https://html.spec.whatwg.org/multipage/web-messaging.html#message-ports:handler-messageeventtarget-onmessage
+    // The first time a MessagePort object's onmessage IDL attribute is set, the port's port message queue must be enabled,
+    // as if the start() method had been called.
+    m_internal_port->start();
+}
+
+void DedicatedWorkerGlobalScope::set_onmessageerror(WebIDL::CallbackType* callback)
+{
+    set_event_handler_attribute(EventNames::messageerror, callback);
+}
+
+WebIDL::CallbackType* DedicatedWorkerGlobalScope::onmessageerror()
+{
+    return event_handler_attribute(EventNames::messageerror);
+}
 
 }