Implement CSSOM Smooth Scroll API

Source/platform/scroll/ScrollView.cpp

Index: Source/platform/scroll/ScrollView.cpp
diff --git a/Source/platform/scroll/ScrollView.cpp b/Source/platform/scroll/ScrollView.cpp
index 56bc53593a29dea66d28b8b11cab74951a4a420a..ba3958575ade7bf08a1a9fe60493947931dc976b 100644
--- a/Source/platform/scroll/ScrollView.cpp
+++ b/Source/platform/scroll/ScrollView.cpp
@@ -279,14 +279,17 @@ void ScrollView::scrollTo(const IntSize& newOffset)
         scrollContents(scrollDelta);
 }
 
-void ScrollView::setScrollPosition(const IntPoint& scrollPoint)
+void ScrollView::setScrollPosition(const IntPoint& scrollPoint, ScrollBehavior scrollBehavior)
 {
     IntPoint newScrollPosition = adjustScrollPositionWithinRange(scrollPoint);
 
     if (newScrollPosition == scrollPosition())
         return;
 
-    updateScrollbars(IntSize(newScrollPosition.x(), newScrollPosition.y()));
+    if (scrollBehavior == ScrollBehaviorInstant)
+        updateScrollbars(IntSize(newScrollPosition.x(), newScrollPosition.y()));
+    else
+        ScrollableArea::programmaticallyScrollSmoothlyToOffset(newScrollPosition);
 }
 
 bool ScrollView::scroll(ScrollDirection direction, ScrollGranularity granularity)