fix(paper-select): positioning when content is scrollable

[panthony]

This PRs fix #773.

The issue was that at the time where scrollHeight and offsetHeight were retrieved the container was still hidden.

Because of this both values where always 0.

Moving the statement below fixes the issue because the computePosition manually set display:block  on the container.

    // Get the selectMenuRect *after* max-width is possibly set above
    containerNode.style.display = 'block';
    let selectMenuRect = selectNode.getBoundingClientRect();
    let centeredRect = getOffsetRect(centeredNode);

[miguelcobain]

Thanks @panthony. Nice find!