Disable scrollbars on the root scroller when using visual viewport scrollbars.

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 104 matching lines @@
   void mouseCapturedScrollbar();
   void mouseReleasedScrollbar();
   void contentAreaDidShow() const;
   void contentAreaDidHide() const;
 
   void finishCurrentScrollAnimations() const;
 
   virtual void didAddScrollbar(Scrollbar&, ScrollbarOrientation);
   virtual void willRemoveScrollbar(Scrollbar&, ScrollbarOrientation);
 
+  // Called when this ScrollableArea becomes or unbecomes the global root
+  // scroller.
+  virtual void didChangeGlobalRootScroller() {}
+
   virtual void contentsResized();
 
   bool hasOverlayScrollbars() const;
   void setScrollbarOverlayColorTheme(ScrollbarOverlayColorTheme);
   void recalculateScrollbarOverlayColorTheme(Color);
   ScrollbarOverlayColorTheme getScrollbarOverlayColorTheme() const {
     return static_cast<ScrollbarOverlayColorTheme>(
         m_scrollbarOverlayColorTheme);
   }
 
@@ skipping 305 matching lines @@
   // vertical-lr / ltr            NO                      NO
   // vertical-lr / rtl            NO                      YES
   // vertical-rl / ltr            YES                     NO
   // vertical-rl / rtl            YES                     YES
   IntPoint m_scrollOrigin;
 };
 
 }  // namespace blink
 
 #endif  // ScrollableArea_h