Make rootScroller the layout viewport.

third_party/WebKit/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 149 matching lines @@
 class StyleEngine;
 class StyleResolver;
 class StyleSheet;
 class StyleSheetList;
 class Text;
 class TextAutosizer;
 class Touch;
 class TouchList;
 class TransformSource;
 class TreeWalker;
+class ViewportScrollCallback;
 class VisitedLinkState;
 class VisualViewport;
 class WebGLRenderingContext;
 enum class SelectionBehaviorOnFocus;
 struct AnnotatedRegionValue;
 struct FocusParams;
 struct IconURL;
 
 using MouseEventWithHitTestResults = EventWithHitTestResults<PlatformMouseEvent>;
 using ExceptionCode = int;
@@ skipping 896 matching lines @@
 
     void enforceInsecureRequestPolicy(WebInsecureRequestPolicy);
 
     bool mayContainV0Shadow() const { return m_mayContainV0Shadow; }
 
     ShadowCascadeOrder shadowCascadeOrder() const { return m_shadowCascadeOrder; }
     void setShadowCascadeOrder(ShadowCascadeOrder);
 
     bool containsV1ShadowTree() const { return m_shadowCascadeOrder == ShadowCascadeOrder::ShadowCascadeV1; }
 
+    void initializeRootScroller(ViewportScrollCallback*);
     Element* rootScroller() const;
     void setRootScroller(Element*, ExceptionState&);
     const Element* effectiveRootScroller() const;
 
     // TODO(bokan): Temporarily added to allow ScrollCustomization to properly
     // opt out for wheel scrolls. crbug.com/623079.
     bool isViewportScrollCallback(const ScrollStateCallback*);
 
     bool isInMainFrame() const;
 
@@ skipping 363 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h