Implement CSSOM Smooth Scroll API

Source/core/rendering/compositing/RenderLayerCompositor.cpp

Index: Source/core/rendering/compositing/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/compositing/RenderLayerCompositor.cpp b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
index 69fa42c6204dab39b73542a8e0ee3fca55ea996a..8492899f6ae2a46950534b8f5f4b85de67108c39 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -994,6 +994,7 @@ void RenderLayerCompositor::ensureRootLayer()
         m_scrollLayer->addChild(m_rootContentLayer.get());
 
         frameViewDidChangeSize();
+        m_renderView.frameView()->layerForScrollingDidChange();
     }
 
     // Check to see if we have to change the attachment
@@ -1044,6 +1045,7 @@ void RenderLayerCompositor::destroyRootLayer()
         m_overflowControlsHostLayer = nullptr;
         m_containerLayer = nullptr;
         m_scrollLayer = nullptr;
+        m_renderView.frameView()->layerForScrollingDidChange();
     }
     ASSERT(!m_scrollLayer);
     m_rootContentLayer = nullptr;