Fix smooth scroll overshooting when mouse held down in scrollbar track.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.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:
  *
  * 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
@@ skipping 27 matching lines @@
     enum HitTestBehavior { AllowHitTest, DisallowHitTest };
 
     ScrollbarThemeOverlay(int thumbThickness, int scrollbarMargin, HitTestBehavior);
     ScrollbarThemeOverlay(int thumbThickness, int scrollbarMargin, HitTestBehavior, Color);
     ~ScrollbarThemeOverlay() override {}
 
     int scrollbarThickness(ScrollbarControlSize) override;
     int scrollbarMargin() const override;
     bool usesOverlayScrollbars() const override;
 
-    int thumbPosition(const ScrollbarThemeClient&) override;
+    int thumbPosition(const ScrollbarThemeClient&, float scrollPosition) override;
     int thumbLength(const ScrollbarThemeClient&) override;
 
     bool hasButtons(const ScrollbarThemeClient&) override { return false; }
     bool hasThumb(const ScrollbarThemeClient&) override;
 
     IntRect backButtonRect(const ScrollbarThemeClient&, ScrollbarPart, bool painting = false) override;
     IntRect forwardButtonRect(const ScrollbarThemeClient&, ScrollbarPart, bool painting = false) override;
     IntRect trackRect(const ScrollbarThemeClient&, bool painting = false) override;
     int thumbThickness(const ScrollbarThemeClient&) override;
     int thumbThickness() { return m_thumbThickness; }
 
     void paintThumb(GraphicsContext&, const ScrollbarThemeClient&, const IntRect&) override;
     ScrollbarPart hitTest(const ScrollbarThemeClient&, const IntPoint&) override;
 
     static ScrollbarThemeOverlay& mobileTheme();
 
 private:
     int m_thumbThickness;
     int m_scrollbarMargin;
     HitTestBehavior m_allowHitTest;
     Color m_color;
     const bool m_useSolidColor;
 };
 
 } // namespace blink
 
 #endif