Implement Document.scrollingElement API behind experimental feature flag

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 302 matching lines @@
     PassRefPtrWillBeRawPtr<Attr> createAttributeNS(const AtomicString& namespaceURI, const AtomicString& qualifiedName, ExceptionState&, bool shouldIgnoreNamespaceChecks = false);
     PassRefPtrWillBeRawPtr<Node> importNode(Node* importedNode, bool deep, ExceptionState&);
     PassRefPtrWillBeRawPtr<Element> createElementNS(const AtomicString& namespaceURI, const AtomicString& qualifiedName, ExceptionState&);
     PassRefPtrWillBeRawPtr<Element> createElement(const QualifiedName&, bool createdByParser);
 
     bool regionBasedColumnsEnabled() const;
 
     Element* elementFromPoint(int x, int y) const;
     Vector<Element*> elementsFromPoint(int x, int y) const;
     PassRefPtrWillBeRawPtr<Range> caretRangeFromPoint(int x, int y);
+    Element* scrollingElement();
 
     String readyState() const;
 
     String defaultCharset() const;
 
     AtomicString charset() const { return Document::encodingName(); }
     AtomicString characterSet() const { return Document::encodingName(); }
 
     AtomicString encodingName() const;
 
@@ skipping 1117 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h