LibWeb+WebWorker: Move worker execution into a new WebWorker process

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>

Userland/Services/WebWorker/ConnectionFromClient.cpp

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2023, Andrew Kaster <akaster@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <WebWorker/ConnectionFromClient.h>
+#include <WebWorker/DedicatedWorkerHost.h>
+#include <WebWorker/PageHost.h>
+
+namespace WebWorker {
+
+void ConnectionFromClient::die()
+{
+    // FIXME: Do something here (shutdown process/script gracefully?)
+}
+
+void ConnectionFromClient::request_file(Web::FileRequest request)
+{
+    // FIXME: Route this to FSAS or Brower chrome as appropriate instead of allowing
+    //        the WebWorker process filesystem access
+    auto path = request.path();
+    auto request_id = ++last_id;
+
+    m_requested_files.set(request_id, move(request));
+
+    auto file = Core::File::open(path, Core::File::OpenMode::Read);
+
+    if (file.is_error())
+        handle_file_return(file.error().code(), {}, request_id);
+    else
+        handle_file_return(0, IPC::File(*file.value()), request_id);
+}
+
+ConnectionFromClient::ConnectionFromClient(NonnullOwnPtr<Core::LocalSocket> socket)
+    : IPC::ConnectionFromClient<WebWorkerClientEndpoint, WebWorkerServerEndpoint>(*this, move(socket), 1)
+    , m_page_host(PageHost::create(*this))
+{
+}
+
+ConnectionFromClient::~ConnectionFromClient() = default;
+
+Web::Page& ConnectionFromClient::page()
+{
+    return m_page_host->page();
+}
+
+Web::Page const& ConnectionFromClient::page() const
+{
+    return m_page_host->page();
+}
+
+void ConnectionFromClient::start_dedicated_worker(AK::URL const& url, String const& type, String const&, String const&, IPC::File const&)
+{
+    m_worker_host = make_ref_counted<DedicatedWorkerHost>(page(), url, type);
+
+    m_worker_host->run();
+}
+
+void ConnectionFromClient::handle_file_return(i32 error, Optional<IPC::File> const& file, i32 request_id)
+{
+    auto file_request = m_requested_files.take(request_id);
+
+    VERIFY(file_request.has_value());
+    VERIFY(file_request.value().on_file_request_finish);
+
+    file_request.value().on_file_request_finish(error != 0 ? Error::from_errno(error) : ErrorOr<i32> { file->take_fd() });
+}
+
+}