Pass data prop instead of props to _updateViewableItems in _onScroll

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

Libraries/Lists/VirtualizedList.js

@@ -734,7 +734,7 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     const dOffset = offset - this._scrollMetrics.offset;
     const velocity = dOffset / dt;
     this._scrollMetrics = {contentLength, dt, dOffset, offset, timestamp, velocity, visibleLength};
-    this._updateViewableItems(this.props);
+    this._updateViewableItems(this.props.data);
     if (!this.props) {
       return;
     }