Implement Document.scrollingElement API behind experimental feature flag

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index 48512865c6051b24c288d7d512a12251e1fe3d38..7b2a691e1fc7a75d1dcab3e0bb5ce0b30ffd6760 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -1574,7 +1574,7 @@ void FrameView::setScrollPosition(const DoublePoint& scrollPoint, ScrollBehavior
         return;
 
     if (scrollBehavior == ScrollBehaviorAuto) {
-        Element* scrollElement = RuntimeEnabledFeatures::scrollTopLeftInteropEnabled() ? m_frame->document()->documentElement() : m_frame->document()->body();
+        Element* scrollElement = m_frame->document()->scrollingElement();
         LayoutObject* renderer = scrollElement ? scrollElement->layoutObject() : nullptr;
         if (renderer && renderer->style()->scrollBehavior() == ScrollBehaviorSmooth)
             scrollBehavior = ScrollBehaviorSmooth;