Run CSSOM smooth scroll animations on the compositor when possible

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 7bcca104840e3fa4e037b568b9e083fb73331529..b40510fbe92deb7717f7a7444c66dbeeabf288c5 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -210,6 +210,12 @@ void RenderLayerCompositor::updateIfNeededRecursive()
 
     DocumentAnimations::startPendingAnimations(m_renderView.document());
 
+    m_renderView.frameView()->updateCompositorScrollAnimations();
+    if (const FrameView::ScrollableAreaSet* animatingScrollableAreas = m_renderView.frameView()->animatingScrollableAreas()) {
+        for (ScrollableArea* scrollableArea : *animatingScrollableAreas)
+            scrollableArea->updateCompositorScrollAnimations();
+    }
+
 #if ENABLE(ASSERT)
     ASSERT(lifecycle().state() == DocumentLifecycle::CompositingClean);
     assertNoUnresolvedDirtyBits();