Clear cell cache when external scrolling ends

[leoasis]

With the fix in 9.5.0 for using the cell cache inWindowScroller (https://github.com/bvaughn/react-virtualized/blob/master/CHANGELOG.md#950), another bug was unveiled:

Since the Grid when used with WindowScroller does not emit onScroll events when the user scrolls, the cell cache is never cleaned up because the _debounceScrollEnded function is never called.

This PR fixes that by detecting if props.isScrolling changed from true to false, and call this._debounceScrollEnded() in that case.

[bvaughn]

Thanks for this contribution! Released as 9.7.3

[leoasis]

Thanks for accepting it and for this library!