Add overflow:auto scrollbars to child flex basis.

Source/core/paint/DeprecatedPaintLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 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 131 matching lines @@
     void scrollToYOffset(double y, ScrollOffsetClamping clamp = ScrollOffsetUnclamped, ScrollBehavior scrollBehavior = ScrollBehaviorInstant)
     {
         scrollToOffset(DoubleSize(scrollXOffset(), y), clamp, scrollBehavior);
     }
 
     void setScrollPosition(const DoublePoint& position, ScrollType scrollType, ScrollBehavior scrollBehavior = ScrollBehaviorInstant) override
     {
         scrollToOffset(toDoubleSize(position), ScrollOffsetClamped, scrollBehavior);
     }
 
+    void updateScrollDimensions(DoubleSize& scrollOffset, bool& autoHorizontalScrollBarChanged, bool& autoVerticalScrollBarChanged);
+    void finalizeScrollDimensions(const DoubleSize& originalScrollOffset, bool autoHorizontalScrollBarChanged, bool autoVerticalScrollBarChanged);
+
     void updateAfterLayout();
     void updateAfterStyleChange(const ComputedStyle*);
     void updateAfterOverflowRecalc();
 
     bool updateAfterCompositingChange() override;
 
     bool hasScrollbar() const { return m_hBar || m_vBar; }
 
     LayoutScrollbarPart* scrollCorner() const { return m_scrollCorner; }
 
@@ skipping 140 matching lines @@
     LayoutScrollbarPart* m_resizer;
 
 #if ENABLE(ASSERT)
     bool m_hasBeenDisposed;
 #endif
 };
 
 } // namespace blink
 
 #endif // LayerScrollableArea_h