Implement CSSOM Smooth Scroll API

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 3d0f7af4f44e5c713cdce52835231134f8dc2777..4cc9e2fd06e1f30a61a9831bdb9a7eafbf757556 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -380,6 +380,7 @@ bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* sc
             setupScrollbarLayer(verticalScrollbarLayer, scrollbarLayer, webLayer, containerLayer);
     }
 
+    scrollableArea->layerForScrollingDidChange();
     return !!webLayer;
 }
 
@@ -637,7 +638,11 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
 {
     if (WebLayer* scrollLayer = scrollingWebLayerForScrollableArea(m_page->mainFrame()->view())) {
         m_lastMainThreadScrollingReasons = reasons;
-        scrollLayer->setShouldScrollOnMainThread(reasons);
+        bool shouldScrollOnMainThread = reasons;
+        if (shouldScrollOnMainThread != scrollLayer->shouldScrollOnMainThread()) {
+            scrollLayer->setShouldScrollOnMainThread(reasons);
+            m_page->mainFrame()->view()->requiresMainThreadScrollingDidChange();
+        }
     }
 }