Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarThemeMacCommon.h

 /*
  * Copyright (C) 2008 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 19 matching lines @@
 #include "platform/scroll/ScrollbarTheme.h"
 
 namespace WebCore {
 
 class PLATFORM_EXPORT ScrollbarThemeMacCommon : public ScrollbarTheme {
 public:
     virtual ~ScrollbarThemeMacCommon();
 
     virtual void registerScrollbar(ScrollbarThemeClient*) OVERRIDE;
     virtual void unregisterScrollbar(ScrollbarThemeClient*) OVERRIDE;
-    void preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, bool jumpOnTrackClick, NSScrollerStyle preferredScrollerStyle, bool redraw);
+    void preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw);
 
     virtual bool supportsControlTints() const OVERRIDE { return true; }
 
     virtual double initialAutoscrollTimerDelay() OVERRIDE;
     virtual double autoscrollTimerDelay() OVERRIDE;
 
     virtual void paintOverhangBackground(GraphicsContext*, const IntRect& horizontalOverhangArea, const IntRect& verticalOverhangArea, const IntRect& dirtyRect) OVERRIDE;
     virtual void paintOverhangShadows(GraphicsContext*, const IntSize& scrollOffset, const IntRect& horizontalOverhangArea, const IntRect& verticalOverhangArea, const IntRect& dirtyRect) OVERRIDE;
     virtual void paintTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&) OVERRIDE;
 
     static NSScrollerStyle recommendedScrollerStyle();
 
     static bool isOverlayAPIAvailable();
 
 protected:
     virtual int maxOverlapBetweenPages() OVERRIDE { return 40; }
 
-    virtual bool shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) OVERRIDE;
     virtual bool shouldDragDocumentInsteadOfThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) OVERRIDE;
     int scrollbarPartToHIPressedState(ScrollbarPart);
 
     virtual void updateButtonPlacement() { }
 
     void paintGivenTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&, const Vector<IntRect>&);
 
     RefPtr<Pattern> m_overhangPattern;
 };
 
 }
 
 #endif // ScrollbarThemeMacCommon_h