This aligns Workers and Window and MessagePorts to all use the same
mechanism for transferring serialized messages across realms.
It also allows transferring more message ports into a worker.
Re-enable the Worker-echo test, as none of the MessagePort tests have
themselves been flaky, and those are now using the same underlying
implementation.
We now create a WorkerAgent for the parent context, which is currently
only a Window. Note that Workers can have Workers per the spec.
The WorkerAgent spawns a WebWorker process to hold the actual
script execution of the Worker. This is modeled with the
DedicatedWorkerHost object in the WebWorker process.
A start_dedicated_worker IPC method in the WebWorker IPC creates the
WorkerHost object. Future different worker types may use different IPC
messages to create their WorkerHost instance.
This implementation cannot yet postMessage between the parent and the
child processes.
Co-Authored-By: Andreas Kling <kling@serenityos.org>
