Add warning when scrollRef does not have a scrollTo method
Add warning when scrollRef does not have a scrollTo method

Summary:
Add a `console.warn()` call when calling `_scrollRef.scrollTo`, because `scrollTo` is not guaranteed to exist on `_scrollRef`.

Context:
`VirtualizedList` holds `_scrollRef`, which is usually a reference to a `ScrollView` component. However, there are several cases where it holds a `View` or other type of component instead.

A custom component can be passed in to `renderScrollComponent`, and then `_scrollRef` will point to that custom component. Additionally, if two VirtualizedLists are nested with the same orientation, `_defaultRenderScrollComponent` will return a View instead of a ScrollView.

Due to these possibilities, `_scrollRef` is not guaranteed to have a `scrollTo` method.

Changelog: [General] [Added] - Add warning when scrollRef does not have a scrollTo method

Reviewed By: JoshuaGross, TheSavior

Differential Revision: D21386842

fbshipit-source-id: 01e167e0ae0edea8f29853e8b242ce88a5103b49

javache/react-native,javache/react-native,hoangpham95/react-native,hoangpham95/react-native,arthuralee/react-native,myntra/react-native,facebook/react-native,javache/react-native,pandiaraj44/react-native,myntra/react-native,pandiaraj44/react-native,myntra/react-native,javache/react-native,janicduplessis/react-native,hoangpham95/react-native,facebook/react-native,javache/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,arthuralee/react-native,arthuralee/react-native,pandiaraj44/react-native,pandiaraj44/react-native,myntra/react-native,facebook/react-native,facebook/react-native,facebook/react-native,arthuralee/react-native,janicduplessis/react-native,pandiaraj44/react-native,myntra/react-native,janicduplessis/react-native,janicduplessis/react-native,myntra/react-native,javache/react-native,javache/react-native,javache/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,hoangpham95/react-native,pandiaraj44/react-native,facebook/react-native,pandiaraj44/react-native,facebook/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,hoangpham95/react-native,hoangpham95/react-native,janicduplessis/react-native,pandiaraj44/react-native,janicduplessis/react-native