LibWeb: Make Fetch::Infrastructure::{Request,Response,HeaderList} GC'd

This is the way.

On a more serious note, there's no reason to keep adding ref-counted
classes to LibWeb now that the majority of classes is GC'd - it only
adds the risk of discovering some cycle down the line, and forces us to
use handles as we can't visit().

Userland/Libraries/LibWeb/HTML/BrowsingContextContainer.h

@@ -37,7 +37,7 @@ protected:
     void shared_attribute_processing_steps_for_iframe_and_frame(bool initial_insertion);
 
     // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#navigate-an-iframe-or-frame
-    void navigate_an_iframe_or_frame(NonnullRefPtr<Fetch::Infrastructure::Request>);
+    void navigate_an_iframe_or_frame(JS::NonnullGCPtr<Fetch::Infrastructure::Request>);
 
     void create_new_nested_browsing_context();