WebContent: Add shutdown_server IPC call to request process exit

This ensures that removing the last view from a WebContentClient will
close its associated process, assuming the WebContent process is not
hung. A more drastic measure will be needed to trigger forcefully
killing the process when it doesn't respond to this request.

Userland/Libraries/LibWebView/WebContentClient.cpp

@@ -50,6 +50,7 @@ void WebContentClient::unregister_view(u64 page_id)
     m_views.remove(page_id);
     if (m_views.is_empty()) {
         on_web_content_process_crash = nullptr;
+        async_close_server();
     }
 }