Summary:
Existing code may pass negative values for `initialScrollIndex`, which the previous implemnentation handled gracefully. Handle the case gracefully in VirtualizedList_EXPERIMENTAL as well.
Changelog:
[Internal][Fixed] - Gracefully handle negative initialScrollIndex in VirtualizedList_EXPERIMENTAL
Reviewed By: rshest
Differential Revision: D38183625
fbshipit-source-id: 9aea91c4cf3ce2190d769f34ce728a109efc88d4