Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarTheme.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 66 matching lines @@
     }
 
     virtual void invalidatePart(ScrollbarThemeClient*, ScrollbarPart);
 
     virtual void paintScrollCorner(GraphicsContext*, const IntRect& cornerRect);
 
     virtual void paintTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&) { }
     virtual void paintOverhangBackground(GraphicsContext*, const IntRect&, const IntRect&, const IntRect&);
     virtual void paintOverhangShadows(GraphicsContext*, const IntSize&, const IntRect&, const IntRect&, const IntRect&) { }
 
-    virtual bool shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) { return false; }
-    virtual bool shouldSnapBackToDragOrigin(ScrollbarThemeClient*, const PlatformMouseEvent&) { return false; }
+    virtual bool shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent&);
+    virtual bool shouldSnapBackToDragOrigin(ScrollbarThemeClient*, const PlatformMouseEvent&);
     virtual bool shouldDragDocumentInsteadOfThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) { return false; }
 
     // The position of the thumb relative to the track.
     virtual int thumbPosition(ScrollbarThemeClient*);
     // The length of the thumb along the axis of the scrollbar.
     virtual int thumbLength(ScrollbarThemeClient*);
     // The position of the track relative to the scrollbar.
     virtual int trackPosition(ScrollbarThemeClient*);
     // The length of the track along the axis of the scrollbar.
     virtual int trackLength(ScrollbarThemeClient*);
@@ skipping 34 matching lines @@
     static void setMockScrollbarsEnabled(bool flag);
     static bool mockScrollbarsEnabled();
 
 private:
     static ScrollbarTheme* nativeTheme(); // Must be implemented to return the correct theme subclass.
     static bool gMockScrollbarsEnabled;
 };
 
 }
 #endif