LibWeb: Make HTML::Environment a GC-allocated type

The only subclass was already GC-allocated, so let's hoist the JS::Cell
inheritance up one level. This ends up simplifying a bit of rather
dubious looking code where we were previously slicing ESOs.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -263,13 +263,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Document>> Document::create_and_initialize(
             browsing_context->page(),
             creation_url.value(),
             move(realm_execution_context),
-            navigation_params.reserved_environment.visit(
-                // FIXME: Environment is virtual. We *really* shouldn't be slicing it here
-                [](Empty const&) -> Optional<HTML::Environment> { return {}; },
-                [](HTML::Environment* env) -> Optional<HTML::Environment> { if (env) return *env; return {}; },
-                [](JS::NonnullGCPtr<HTML::EnvironmentSettingsObject>) -> Optional<HTML::Environment> {
-                    TODO();
-                }),
+            navigation_params.reserved_environment,
             top_level_creation_url.value(),
             top_level_origin);
     }