fix flow suppression in VirtualizedList
fix flow suppression in VirtualizedList

Summary:
flow 0.146 became more conservative about refinement invalidation, no longer accepting that `this.props.onViewableItemsChanged` is truthy. this was suppressed at the time.

instead, we can assign to a const to restore typechecking.

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D33743126

fbshipit-source-id: 0b1f0b83c2fe812e88b027c3b1d3d8aca7b09277

janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,javache/react-native