Update PaintLayer size during layout, not after.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.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 279 matching lines @@
   IntRect ScrollableAreaBoundingBox() const override;
   void RegisterForAnimation() override;
   void DeregisterForAnimation() override;
   bool UserInputScrollable(ScrollbarOrientation) const override;
   bool ShouldPlaceVerticalScrollbarOnLeft() const override;
   int PageStep(ScrollbarOrientation) const override;
   ScrollBehavior ScrollBehaviorStyle() const override;
   CompositorAnimationHost* GetCompositorAnimationHost() const override;
   CompositorAnimationTimeline* GetCompositorAnimationTimeline() const override;
 
+  LayoutSize ClientSize() const;
+  IntSize PixelSnappedClientSize() const;
+
   void VisibleSizeChanged();
 
   // FIXME: We shouldn't allow access to m_overflowRect outside this class.
   LayoutRect OverflowRect() const { return overflow_rect_; }
 
   void ScrollToAbsolutePosition(
       const FloatPoint& position,
       ScrollBehavior scroll_behavior = kScrollBehaviorInstant,
       ScrollType scroll_type = kProgrammaticScroll) {
     SetScrollOffset(position - ScrollOrigin(), scroll_type, scroll_behavior);
@@ skipping 291 matching lines @@
 
 DEFINE_TYPE_CASTS(PaintLayerScrollableArea,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->IsPaintLayerScrollableArea(),
                   scrollableArea.IsPaintLayerScrollableArea());
 
 }  // namespace blink
 
 #endif  // LayerScrollableArea_h