res:dock:: fix flash problem.

given all the window of a docked client are closed, the icon on fashion mode
will flash.

reason:
set display to none then set display to emtpy which is delayed 10ms, otherwise it
won't work.

solution:
using offsetWidth property to force webkit to calculate to avoid to be delayed.

Change-Id: I1f0493e6fafc50015eabbb87a25b95ca990122e9