Use Math.ceil() when computing a modalbox's position.

modalBox would listen for updates to the content within the box and
attempt to update its size and position accordingly. These values could
end up being non-integers, causing elements to subtly jump.

To fix this, we just need to use Math.ceil() on the values to ensure
that we get whole numbers.

Reviewed at https://reviews.reviewboard.org/r/6162/