Initial draft - modify ViewportAnchor to know about both inner and outer viewports.

Source/platform/scroll/ScrollView.h

Index: Source/platform/scroll/ScrollView.h
diff --git a/Source/platform/scroll/ScrollView.h b/Source/platform/scroll/ScrollView.h
index 851fea1fa1e90fb477a14cc3a36f3a6c3fbe8a08..2aeca67d60410f209ae32b0bf89cf13141ba07fd 100644
--- a/Source/platform/scroll/ScrollView.h
+++ b/Source/platform/scroll/ScrollView.h
@@ -55,9 +55,6 @@ public:
 
     virtual void notifyPageThatContentAreaWillPaint() const;
 
-    // NOTE: This should only be called by the overriden setScrollOffset from ScrollableArea.
-    virtual void scrollTo(const IntSize& newOffset);
-
     // The window that hosts the ScrollView. The ScrollView will communicate scrolls and repaints to the
     // host window in the window's coordinate space.
     virtual HostWindow* hostWindow() const = 0;
@@ -254,6 +251,9 @@ public:
 protected:
     ScrollView();
 
+    // NOTE: This should only be called by the overriden setScrollOffset from ScrollableArea.
+    virtual void scrollTo(const IntSize& newOffset);
+
     virtual void contentRectangleForPaintInvalidation(const IntRect&);
     virtual void paintContents(GraphicsContext*, const IntRect& damageRect) = 0;