Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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 226 matching lines @@
     virtual GraphicsLayer* layerForHorizontalScrollbar() const { return 0; }
     virtual GraphicsLayer* layerForVerticalScrollbar() const { return 0; }
     virtual GraphicsLayer* layerForScrollCorner() const { return 0; }
     bool hasLayerForHorizontalScrollbar() const;
     bool hasLayerForVerticalScrollbar() const;
     bool hasLayerForScrollCorner() const;
 
     void layerForScrollingDidChange(WebCompositorAnimationTimeline*);
 
     void cancelScrollAnimation();
-    void cancelProgrammaticScrollAnimation();
+    virtual void cancelProgrammaticScrollAnimation();
 
     virtual ~ScrollableArea();
 
     // Called when any of horizontal scrollbar, vertical scrollbar and scroll corner is setNeedsPaintInvalidation.
     virtual void scrollControlWasSetNeedsPaintInvalidation() = 0;
 
     // Returns the default scroll style this area should scroll with when not
     // explicitly specified. E.g. The scrolling behavior of an element can be
     // specified in CSS.
     virtual ScrollBehavior scrollBehaviorStyle() const { return ScrollBehaviorInstant; }
@@ skipping 15 matching lines @@
     // Does nothing if overlay scrollbars are enabled.
     IntSize excludeScrollbars(const IntSize&) const;
 
     // Need to promptly let go of owned animator objects.
     EAGERLY_FINALIZE();
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     ScrollableArea();
 
+    ScrollbarOrientation scrollbarOrientationFromDirection(ScrollDirectionPhysical) const;
+    float scrollStep(ScrollGranularity, ScrollbarOrientation) const;
+
     void setScrollOrigin(const IntPoint&);
     void resetScrollOriginChanged() { m_scrollOriginChanged = false; }
 
     // Needed to let the animators call scrollPositionChanged.
     friend class ScrollAnimatorBase;
     friend class ProgrammaticScrollAnimator;
     void scrollPositionChanged(const DoublePoint&, ScrollType);
 
     void clearScrollAnimators();
 
@@ skipping 52 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