Background jitters on pages with background-attachment: fixed

Background jitters on pages with background-attachment: fixed
https://bugs.webkit.org/show_bug.cgi?id=79182

Reviewed by Dan Bernstein.

Make sure to update the main frame scroll position before updating the scrolling layer.
This also fixes a (purely theoretical) bug where we wouldn't be updating the main frame
position if the main frame didn't have a scroll layer.

* page/scrolling/ScrollingCoordinator.cpp:
(WebCore::ScrollingCoordinator::updateMainFrameScrollPositionAndScrollLayerPosition):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108434 268f45cc-cd09-0410-ab3c-d52691b4dbfc