Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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

 /*
  * Copyright (c) 2011, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 27 matching lines @@
 
 namespace blink {
 
 class ScrollAnimatorTest;
 
 class PLATFORM_EXPORT ScrollAnimator final : public ScrollAnimatorBase {
 public:
     explicit ScrollAnimator(ScrollableArea*, WTF::TimeFunction = WTF::monotonicallyIncreasingTime);
     ~ScrollAnimator() override;
 
+    float computeDeltaToConsume(ScrollbarOrientation, float pixelDelta) const override;
+
     ScrollResultOneDimensional userScroll(ScrollbarOrientation, ScrollGranularity, float step, float delta) override;
     void scrollToOffsetWithoutAnimation(const FloatPoint&) override;
 
     void cancelAnimations() override;
     void serviceScrollAnimations() override;
     bool hasRunningAnimation() const override;
 
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     void animationTimerFired();
 
     OwnPtr<WebScrollOffsetAnimationCurve> m_animationCurve;
     double m_startTime;
     WTF::TimeFunction m_timeFunction;
+private:
+    FloatPoint desiredTargetPosition() const;
 };
 
 } // namespace blink
 
 #endif // ScrollAnimator_h