During Serial full-gc marking, `StrongRootsScope` is used to prepare nmethod marking and thread-claim. Since Serial uses single thread for GC, the thread-claim part is statically skipped by passing zero in `StrongRootsScope srs(0)`.

In order to make it more explicit what `StrongRootsScope` entails in this context, one can probably use something more precise to convey what's expected in this context, e.g. NMethodMarkingScope.