We're seeing runtime crashes at [WKWebViewIOS _didFinishScrolling] in situations where a web view is closed out during a scroll operation.

This regression surfaced from https://commits.webkit.org/260975@main because it (correctly) changed the relative order of destruction between the `DrawingAreaProxy` and the `RemoteScrollingCoordinatorProxy` (and the `RemoteScrollingTree` it encompasses), which meant that there could be situations where closing or switching out a web view in the middle of a scroll operation could lead to a null deref of the `RemoteScrollingCoordinatorProxy` held by the `WebPageProxy`.