Implement CSSOM Smooth Scroll API

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 0f7daf4090606791c18d46cc295d95f4976d5924..06463cdc915910423086a2251dffe35cd255d9e3 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -2307,6 +2307,7 @@ void RenderLayerCompositor::ensureRootLayer()
 
         frameViewDidChangeSize();
         frameViewDidScroll();
+        m_renderView->frameView()->layerForScrollingDidChange();
     }
 
     // Check to see if we have to change the attachment
@@ -2357,6 +2358,7 @@ void RenderLayerCompositor::destroyRootLayer()
         m_overflowControlsHostLayer = nullptr;
         m_containerLayer = nullptr;
         m_scrollLayer = nullptr;
+        m_renderView->frameView()->layerForScrollingDidChange();
     }
     ASSERT(!m_scrollLayer);
     m_rootContentLayer = nullptr;