Revert of Remove Overlay Scrollbar if not scrollable along the axis (patchset #9 id:1140001 of http…

Source/core/rendering/RenderLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 598 matching lines @@
     {
         // Hits in compositing/overflow/automatically-opt-into-composited-scrolling-after-style-change.html.
         DisableCompositingQueryAsserts disabler;
 
         // overflow:scroll should just enable/disable.
         if (box().style()->overflowX() == OSCROLL)
             horizontalScrollbar()->setEnabled(hasHorizontalOverflow);
         if (box().style()->overflowY() == OSCROLL)
             verticalScrollbar()->setEnabled(hasVerticalOverflow);
     }
-    if (hasOverlayScrollbars()) {
-        if (!scrollSize(HorizontalScrollbar))
-            setHasHorizontalScrollbar(false);
-        if (!scrollSize(VerticalScrollbar))
-            setHasVerticalScrollbar(false);
-    }
+
     // overflow:auto may need to lay out again if scrollbars got added/removed.
     bool autoHorizontalScrollBarChanged = box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow);
     bool autoVerticalScrollBarChanged = box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow);
 
     if (autoHorizontalScrollBarChanged || autoVerticalScrollBarChanged) {
         if (box().hasAutoHorizontalScrollbar())
             setHasHorizontalScrollbar(hasHorizontalOverflow);
         if (box().hasAutoVerticalScrollbar())
             setHasVerticalScrollbar(hasVerticalOverflow);
 
@@ skipping 856 matching lines @@
 void RenderLayerScrollableArea::setTopmostScrollChild(RenderLayer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink