Update VirtualizedList.js, fix wrong position after scrollToEnd when props.refreshing === true

Summary:
Pull request again for clean branch

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- [x] Provide a **test plan** demonstrating that the code is solid.
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What existing problem does the pull request solve?
Fix #14033
Fix wrong position after `scrollToEnd` when `props.refreshing === true`

**Before**

![before](https://cloud.githubusercontent.com/assets/1185694/26242273/657f0180-3cba-11e7-9360-49269fd1bb2f.gif)

**After**

![after](https://cloud.githubusercontent.com/assets/1185694/26242503/465a856c-3cbb-11e7-98fb-f3256fd45c15.gif)

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
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Closes https://github.com/facebook/react-native/pull/14089

Differential Revision: D5283973

Pulled By: sahrens

fbshipit-source-id: 9bef9a922c15aa5bed11a59acbd7b48a76946d29

Libraries/Lists/VirtualizedList.js

@@ -209,11 +209,13 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     const animated = params ? params.animated : true;
     const veryLast = this.props.getItemCount(this.props.data) - 1;
     const frame = this._getFrameMetricsApprox(veryLast);
-    const offset =
+    const offset = Math.max(
+      0,
       frame.offset +
         frame.length +
         this._footerLength -
-      this._scrollMetrics.visibleLength;
+        this._scrollMetrics.visibleLength,
+    );
     this._scrollRef.scrollTo(
       this.props.horizontal ? {x: offset, animated} : {y: offset, animated},
     );