vector/jquery.ui.core.css: Update rule for .ui-helper-hidden-accessible

A Blink rendering bug causes dialogs to vanish in Chrome and Opera Next
with current CSS (bug 67243).

The file resources/lib/jquery.ui/themes/smoothness/jquery.ui.core.css
(from the default jquery.ui theme, 'smoothness') was used as a template.

Upstream commits:
* https://github.com/jquery/jquery-ui/commit/a9d854967f0
* https://github.com/jquery/jquery-ui/commit/48e0aa0fd27

Bug: 67243
Change-Id: Ib099282484bb29f5d33208be843686bf3ca11366
Merge "vector/jquery.ui.core.css: Update rule for .ui-helper-hidden-accessible"