Correcting horizontal scroll bar in RTL

Instructions:
Install a RTL language (Persian or Arabic) or set en-GB to RTL in en-GB.xml, Set as the admin language.
Display the article manager in a large screen.
There is an horizontal scroll bar showing at bottom and moving it displays white screen at left.
![before](https://cloud.githubusercontent.com/assets/869724/2750237/bc8dbd86-c85f-11e3-967b-e051e9dba6b4.png)

Patch, clean cache.
No more horizontal scroll bar.
![after](https://cloud.githubusercontent.com/assets/869724/2750242/f404c296-c85f-11e3-861e-d0066b38c974.png)

Thanks to our Arabic Translation Coordinator.