fix #678 Work-around for scrollbars not removed correctly on webkit.

Adds an aria.utils.Dom.refreshScrollbars method as a work-around
for Chrome bug http://crbug.com/240772 (this bug also appears on
some other webkit browsers) : scrollbars are not removed when they
are no longer necessary if the content has nearly the same size as
the container.

(cherry picked from commit 661c880dcff141c5a6823dceb5339ab192422821)