Re-add snap-back behavior (Blink side)

Source/platform/scroll/ScrollbarThemeGtkOrAura.cpp

 /*
  * Copyright (c) 2008, 2009, 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 125 matching lines @@
     blink::WebCanvas* canvas = gc->canvas();
     if (scrollbar->pressedPart() == ThumbPart)
         state = blink::WebThemeEngine::StatePressed;
     else if (scrollbar->hoveredPart() == ThumbPart)
         state = blink::WebThemeEngine::StateHover;
     else
         state = blink::WebThemeEngine::StateNormal;
     blink::Platform::current()->themeEngine()->paint(canvas, scrollbar->orientation() == HorizontalScrollbar ? blink::WebThemeEngine::PartScrollbarHorizontalThumb : blink::WebThemeEngine::PartScrollbarVerticalThumb, state, blink::WebRect(rect), 0);
 }
 
-bool ScrollbarThemeGtkOrAura::shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent& evt)
-{
-    return (evt.shiftKey() && evt.button() == LeftButton) || (evt.button() == MiddleButton);
-}
-
 IntSize ScrollbarThemeGtkOrAura::buttonSize(ScrollbarThemeClient* scrollbar)
 {
     if (scrollbar->orientation() == VerticalScrollbar) {
         IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarUpArrow);
         return IntSize(size.width(), scrollbar->height() < 2 * size.height() ? scrollbar->height() / 2 : size.height());
     }
 
     // HorizontalScrollbar
     IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarLeftArrow);
     return IntSize(scrollbar->width() < 2 * size.width() ? scrollbar->width() / 2 : size.width(), size.height());
 }
 
 int ScrollbarThemeGtkOrAura::minimumThumbLength(ScrollbarThemeClient* scrollbar)
 {
     if (scrollbar->orientation() == VerticalScrollbar) {
         IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarVerticalThumb);
         return size.height();
     }
 
     IntSize size = blink::Platform::current()->themeEngine()->getSize(blink::WebThemeEngine::PartScrollbarHorizontalThumb);
     return size.width();
 }
 
 } // namespace WebCore