Implement Document.scrollingElement API behind experimental feature flag

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 130 matching lines @@
     // http://dev.w3.org/csswg/cssom/#extensions-to-the-document-interface
     [SameObject] readonly attribute StyleSheetList styleSheets;
     attribute DOMString? selectedStylesheetSet;
     readonly attribute DOMString? preferredStylesheetSet;
 
     // CSSOM View Module
     // http://dev.w3.org/csswg/cssom-view/#extensions-to-the-document-interface
     // FIXME: The x and y arguments should be of type double.
     Element? elementFromPoint(long x, long y);
     sequence<Element> elementsFromPoint(long x, long y);
+    [RuntimeEnabled=DocumentScrollingElement] readonly attribute Element? scrollingElement;
 
     // Selection API
     // http://w3c.github.io/selection-api/#extensions-to-document-interface
     Selection? getSelection();
 
     // Pointer Lock
     // https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-document-interface
     attribute EventHandler onpointerlockchange;
     attribute EventHandler onpointerlockerror;
     [MeasureAs=DocumentPointerLockElement] readonly attribute Element? pointerLockElement;
@@ skipping 58 matching lines @@
     attribute EventHandler onsearch;
     [RuntimeEnabled=ExperimentalContentSecurityPolicyFeatures] attribute EventHandler onsecuritypolicyviolation;
     attribute EventHandler onselectionchange;
     attribute EventHandler onselectstart;
     attribute EventHandler onwheel;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;
 Document implements NonElementParentNode;