Be able to scroll to last item in list

Summary:
The invariant condition doesn't allow scrolling to the last index

Here is a simple reproduction of the problem: https://snack.expo.io/BJ59gzWrZ

Surely, if the last item is rendered, we should be able to scroll to it without an invariant failing?

Tested locally that this change fixes the issue.
Closes https://github.com/facebook/react-native/pull/14934

Differential Revision: D5414535

Pulled By: shergin

fbshipit-source-id: 38dac4c0e2ae5e1e199a67ca0e98e8f7325e77f4

Libraries/Lists/VirtualizedList.js

@@ -235,7 +235,7 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
       `scrollToIndex out of range: ${index} vs ${getItemCount(data) - 1}`,
     );
     invariant(
-      getItemLayout || index < this._highestMeasuredFrameIndex,
+      getItemLayout || index <= this._highestMeasuredFrameIndex,
       'scrollToIndex should be used in conjunction with getItemLayout, ' +
         'otherwise there is no way to know the location of an arbitrary index.',
     );