Implement Document.scrollingElement API behind experimental feature flag

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1556 matching lines @@
 {
     cancelProgrammaticScrollAnimation();
     TemporaryChange<bool> changeInProgrammaticScroll(m_inProgrammaticScroll, true);
     m_maintainScrollPositionAnchor = nullptr;
 
     DoublePoint newScrollPosition = adjustScrollPositionWithinRange(scrollPoint);
     if (newScrollPosition == scrollPositionDouble())
         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;
         else
             scrollBehavior = ScrollBehaviorInstant;
     }
 
     if (scrollBehavior == ScrollBehaviorInstant) {
         DoubleSize newOffset(newScrollPosition.x(), newScrollPosition.y());
         updateScrollbars(newOffset);
@@ skipping 2516 matching lines @@
 {
     Settings* settings = frame().settings();
     if (!settings || !settings->rootLayerScrolls())
         return this;
 
     LayoutView* layoutView = this->layoutView();
     return layoutView ? layoutView->scrollableArea() : nullptr;
 }
 
 } // namespace blink