Don't overlap test for composited scroll DIVs after scroll
https://bugs.webkit.org/show_bug.cgi?id=107471

Patch by Glenn Hartmann <hartmanng@chromium.org> on 2013-01-29
Reviewed by Simon Fraser.

We don't need to test for overlap after scroll when both
usesCompositedScrolling and !hasOutOfFlowPositionedDescendant
because:

a) Since we're using composited-scrolling, the composited region
presented by the composited-scrolling element to other non-descendant
layers doesn't change during composited scrolling (it's always the
entire scroll layer), and

b) Since we have no out of flow positioned descendants, the scrolling
descendants all move together, so their overlap with respect to each
other cannot change.

So no descendants nor any non-descendants can have their overlap
affected, so it's safe to skip testing.

No new tests (no change in behaviour).

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::updateCompositingLayersAfterScroll):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateCompositingLayers):
* rendering/RenderLayerCompositor.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141121 268f45cc-cd09-0410-ab3c-d52691b4dbfc