[BlackBerry] Fixed background is scrolling in http://www.nieuwecode.nl

[BlackBerry] Fixed background is scrolling in nieuwecode.nl
https://bugs.webkit.org/show_bug.cgi?id=85109

Patch by Arvid Nilsson <anilsson@rim.com> on 2012-04-27
Reviewed by Antonio Gomes.

Since the BlackBerry port uses very similar fixed position acceleration
as the Qt WebKit2 port, the same fix that worked for them in bug 83980
works for us.

Fixed by opting in to the FIXED_POSITION_CREATES_STACKING_CONTEXT
mechanism.

Covered by existing manual test fixed-position-no-z-index.html.

* css/StyleResolver.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@115530 268f45cc-cd09-0410-ab3c-d52691b4dbfc