Change FrameView::scrollContentsFastPath to use m_fixedObjects

Change FrameView::scrollContentsFastPath to use m_fixedObjects
https://bugs.webkit.org/show_bug.cgi?id=90045

Reviewed by James Robinson.

FrameView now has a hash set of fixed-position objects, so use
that instead of RenderBlock::positionedObjects(); we'll avoid traversing
through absolutely positioned objects, and this will work better for sticky
positioning in future.

No behavior change, so no new tests.

* page/FrameView.cpp:
(WebCore::FrameView::scrollContentsFastPath):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121443 268f45cc-cd09-0410-ab3c-d52691b4dbfc