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.h

@@ -10,10 +10,6 @@
 #include <LibWeb/Bindings/WorkerGlobalScopePrototype.h>
 #include <LibWeb/HTML/WorkerGlobalScope.h>
 
-#define ENUMERATE_DEDICATED_WORKER_GLOBAL_SCOPE_EVENT_HANDLERS(E) \
-    E(onmessage, HTML::EventNames::message)                       \
-    E(onmessageerror, HTML::EventNames::messageerror)
-
 namespace Web::HTML {
 
 class DedicatedWorkerGlobalScope
@@ -30,12 +26,11 @@ public:
 
     void close();
 
-#undef __ENUMERATE
-#define __ENUMERATE(attribute_name, event_name)       \
-    void set_##attribute_name(WebIDL::CallbackType*); \
-    WebIDL::CallbackType* attribute_name();
-    ENUMERATE_DEDICATED_WORKER_GLOBAL_SCOPE_EVENT_HANDLERS(__ENUMERATE)
-#undef __ENUMERATE
+    WebIDL::CallbackType* onmessage();
+    void set_onmessage(WebIDL::CallbackType* callback);
+
+    WebIDL::CallbackType* onmessageerror();
+    void set_onmessageerror(WebIDL::CallbackType* callback);
 
     virtual void finalize() override;