Implement CSSOM smooth scroll for Elements

Source/core/rendering/RenderBox.h

Index: Source/core/rendering/RenderBox.h
diff --git a/Source/core/rendering/RenderBox.h b/Source/core/rendering/RenderBox.h
index a4239440a36090760eee8c1fdc5c22a329e523ac..586598f4469318ab6edce40766261ae953fdd1d9 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -27,6 +27,7 @@
 #include "core/rendering/RenderOverflow.h"
 #include "core/rendering/shapes/ShapeOutsideInfo.h"
 #include "platform/scroll/ScrollTypes.h"
+#include "platform/scroll/ScrollableArea.h"
 
 namespace blink {
 
@@ -264,7 +265,7 @@ public:
     virtual void setScrollLeft(LayoutUnit);
     virtual void setScrollTop(LayoutUnit);
 
-    void scrollToOffset(const DoubleSize&);
+    void scrollToOffset(const DoubleSize&, ScrollBehavior = ScrollBehaviorInstant);
     void scrollByRecursively(const DoubleSize& delta, ScrollOffsetClamping = ScrollOffsetUnclamped);
     void scrollRectToVisible(const LayoutRect&, const ScrollAlignment& alignX, const ScrollAlignment& alignY);