Make composited scrolling codepaths co-operate.

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index 6a0a5cf9b979728ba9f7e1918e90e92d1947c4e1..32c2b6ae22b8e9acfa138439388c32286b4eba20 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -335,19 +335,15 @@ void RenderLayerCompositor::updateCompositingRequirementsState()
     if (!rootRenderLayer() || !rootRenderLayer()->acceleratedCompositingForOverflowScrollEnabled())
         return;
 
-    const bool compositorDrivenAcceleratedScrollingEnabled = rootRenderLayer()->compositorDrivenAcceleratedScrollingEnabled();
-
-    const FrameView::ScrollableAreaSet* scrollableAreas = m_renderView->frameView()->scrollableAreas();
-    if (!compositorDrivenAcceleratedScrollingEnabled && !scrollableAreas)
-        return;
-
     for (HashSet<RenderLayer*>::iterator it = m_outOfFlowPositionedLayers.begin(); it != m_outOfFlowPositionedLayers.end(); ++it)
         (*it)->updateHasUnclippedDescendant();
 
-    if (!compositorDrivenAcceleratedScrollingEnabled) {
-        for (FrameView::ScrollableAreaSet::iterator it = scrollableAreas->begin(); it != scrollableAreas->end(); ++it)
-            (*it)->updateNeedsCompositedScrolling();
-    }
+    const FrameView::ScrollableAreaSet* scrollableAreas = m_renderView->frameView()->scrollableAreas();
+    if (!scrollableAreas)
+        return;
+
+    for (FrameView::ScrollableAreaSet::iterator it = scrollableAreas->begin(); it != scrollableAreas->end(); ++it)
+        (*it)->updateNeedsCompositedScrolling();
 }
 
 void RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType updateType, RenderLayer* updateRoot)
@@ -2006,7 +2002,7 @@ bool RenderLayerCompositor::requiresCompositingForBlending(RenderObject* rendere
 
 bool RenderLayerCompositor::requiresCompositingForOverflowScrollingParent(const RenderLayer* layer) const
 {
-    if (!layer->compositorDrivenAcceleratedScrollingEnabled())
+    if (!layer->useCompositorDrivenAcceleratedScrolling())
         return false;
 
     // A layer scrolls with its containing block. So to find the overflow scrolling layer