Audit scroll widgets in DevTools to see if notificationPredicate is needed

A [`notificationPredicate`](https://api.flutter.dev/flutter/material/Scrollbar/notificationPredicate.html) is required for scrollbars whose corresponding scrollview is not the first nested scrollview beneath them (see image below for an example where the`notificationPredicate` is needed). DevTools wasn't using `notificationPredicate` before https://github.com/flutter/devtools/pull/3262. We should check to see if it's needed anywhere else.