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/Fetch/Infrastructure/HTTP/Headers.h

@@ -13,6 +13,8 @@
 #include <AK/Optional.h>
 #include <AK/String.h>
 #include <AK/Vector.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/Cell.h>
 #include <LibWeb/MimeSniff/MimeType.h>
 
 namespace Web::Fetch::Infrastructure {
@@ -29,13 +31,17 @@ struct Header {
 // https://fetch.spec.whatwg.org/#concept-header-list
 // A header list is a list of zero or more headers. It is initially the empty list.
 class HeaderList final
-    : public RefCounted<HeaderList>
+    : public JS::Cell
     , Vector<Header> {
+    JS_CELL(HeaderList, JS::Cell);
+
 public:
     using Vector::begin;
     using Vector::clear;
     using Vector::end;
 
+    [[nodiscard]] static JS::NonnullGCPtr<HeaderList> create(JS::VM&);
+
     [[nodiscard]] bool contains(ReadonlyBytes) const;
     [[nodiscard]] ErrorOr<Optional<ByteBuffer>> get(ReadonlyBytes) const;
     [[nodiscard]] ErrorOr<Optional<Vector<String>>> get_decode_and_split(ReadonlyBytes) const;