Allow custom WindowScroller child with childRef in children function

[TrySound]

findDOMNode is not quite flexible with complex layouts. This PR introduces childRef to improve this. Current behavior uses findDOMNode result until childRef is called

<WindowScroller>
{({ childRef }) => <div ref={childRef} />}
</WindowScroller>

Also bit refactored WindowScroller tests with jest mocks.

[bvaughn]

I have some concerns about this PR.

• The docs have not been updated.
• The childRef name is inconsistent with other components such as InfiniteLoader and ColumnSizer; (they all uses registerChild).