Browser+LibWeb+WebContent: Allow Browser to load local files

To achieve this goal:
 - The Browser unveils "/tmp/portal/filesystemaccess"
 - Pass the page through LoadRequest => ResourceLoader
 - ResourceLoader requests a file to the FileSystemAccessServer via IPC
 - OutOfProcessWebView handles it and sends a file descriptor back to
 the Page.

Userland/Libraries/LibWeb/Loader/ResourceLoader.h

@@ -14,6 +14,7 @@
 #include <LibCore/Object.h>
 #include <LibCore/Proxy.h>
 #include <LibWeb/Loader/Resource.h>
+#include <LibWeb/Page/Page.h>
 
 namespace Web {
 
@@ -98,6 +99,7 @@ private:
     HashTable<NonnullRefPtr<ResourceLoaderConnectorRequest>> m_active_requests;
     NonnullRefPtr<ResourceLoaderConnector> m_connector;
     String m_user_agent;
+    Optional<Page&> m_page {};
 };
 
 }