Refactored `AutoSizer` slightly to add support for `react-lite`.

Note that if you intend to use the UMD build of `react-lite` the following lines are required before `react-virtualized` is loaded:
```js
React.addons = {
  shallowCompare (context, nextProps, nextState) {
    return React.PureComponent.prototype.shouldComponentUpdate(nextProps, nextState)
  }
};
ReactDOM = React;
```
