Summary:
This is a simple bugfix. `_updateViewableItems` expects the `data` prop, as it calls `getItemCount` on it. Without this, `onViewableItemsChanged` updates twice for each scroll position, once from `_onScroll` with the incorrect results, and once from `_updateCellsToRender` with the correct results. This means the callback nigh-on unusable.
I simply logged the results of `onViewableItemsChanged` and made sure they were correct.
Closes https://github.com/facebook/react-native/pull/14162
Differential Revision: D5128363
Pulled By: sahrens
fbshipit-source-id: 3faef812068921a26ed871339da79870fd0d911c
803a9168f2