Tests failing on WK2 bots after giving fixed positioning its own 
stacking context

Reviewed by Geoffrey Garen.

Tests are failing because I checked in updated results that expect 
fixed positioning to create a stacking context. However, 
WebKitTestRunner does not enable ScrollingCoordinator, so the code 
that I added to enable stacking contexts for fixedPos is never 
actually running. This patch moves that code to a place where it will 
run for the testing tools, which matches the patch for WK1.
* WebProcess/WebPage/DrawingAreaImpl.cpp:
(WebKit::DrawingAreaImpl::updatePreferences):
* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::TiledCoreAnimationDrawingArea):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@128678 268f45cc-cd09-0410-ab3c-d52691b4dbfc