LibWeb: Fix some GCVerifier warnings

Author: https://github.com/mattco98
Commit: 8450041b52
Pull-request: https://github.com/SerenityOS/serenity/pull/23861
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/Streams/ReadableStreamDefaultReader.cpp

@@ -148,7 +148,7 @@ private:
     }
 
     JS::NonnullGCPtr<JS::Realm> m_realm;
-    WebIDL::Promise& m_promise;
+    JS::NonnullGCPtr<WebIDL::Promise> m_promise;
 };
 
 // https://streams.spec.whatwg.org/#default-reader-read