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 3ffccdca21d1812740483c8ab056ba7a554163a6..2b8f98231b01cb4056fcb1c03159562bcf18ae61 100644
--- a/Source/platform/scroll/ScrollView.cpp
+++ b/Source/platform/scroll/ScrollView.cpp
@@ -285,14 +285,17 @@ void ScrollView::scrollTo(const IntSize& newOffset)
     updateFixedElementsAfterScrolling();
 }
 
-void ScrollView::setScrollPosition(const IntPoint& scrollPoint)
+void ScrollView::setScrollPosition(const IntPoint& scrollPoint, ScrollBehavior behavior)
 {
     IntPoint newScrollPosition = adjustScrollPositionWithinRange(scrollPoint);
 
     if (newScrollPosition == scrollPosition())
         return;
 
-    updateScrollbars(IntSize(newScrollPosition.x(), newScrollPosition.y()));
+    if (behavior == ScrollBehaviorInstant)
+        updateScrollbars(IntSize(newScrollPosition.x(), newScrollPosition.y()));
+    else
+        ScrollableArea::programmaticallyScrollSmoothlyToOffset(newScrollPosition);
 }
 
 bool ScrollView::scroll(ScrollDirection direction, ScrollGranularity granularity)