Fix scrollLeft/scrollTop warning in latest Chrome

Chrome gives the warnings

```
body.scrollLeft is deprecated in strict mode. Please use 'documentElement.scrollLeft' if in strict mode and 'body.scrollLeft' only if in quirks mode.
body.scrollTop is deprecated in strict mode. Please use 'documentElement.scrollTop' if in strict mode and 'body.scrollTop' only if in quirks mode.
```

the first time getUnboundedScrollPosition is invoked. This fixes it. (All modern browsers support `pageYOffset`; IE 8 doesn't but works properly with `document.documentElement.scrollTop` except in quirks mode which React doesn't support.)
Merge pull request #643 from spicyj/scroll-pos

Fix scrollLeft/scrollTop warning in latest Chrome