Set m_{width,height}AvailableToChildrenChanged when scrollbars appear

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.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@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 663 matching lines @@
 
         // Force an update since we know the scrollbars have changed things.
         if (box().document().hasAnnotatedRegions())
             box().document().setAnnotatedRegionsDirty(true);
 
         // Our proprietary overflow: overlay value doesn't trigger a layout.
         if ((horizontalScrollBarChanged && box().style()->overflowX() != OverflowOverlay) || (verticalScrollBarChanged && box().style()->overflowY() != OverflowOverlay)) {
             if (!m_inOverflowRelayout) {
                 m_inOverflowRelayout = true;
                 if (delayedLayoutScope) {
+                    if (box().isLayoutBlock()) {
+                        LayoutBlock& block = toLayoutBlock(box());
+                        block.scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);

[mstensho (USE GERRIT), 2016/03/18 19:04:26]

I'm not going to complain if you squash this into one line, and get rid of the
curly braces in the process. :)

[cbiesinger, 2016/03/18 19:13:47]

Done.

+                    }
                     delayedLayoutScope->setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
                     didMarkForDelayedLayout = true;
                 } else {
                     SubtreeLayoutScope layoutScope(box());
                     layoutScope.setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
                     if (box().isLayoutBlock()) {
                         LayoutBlock& block = toLayoutBlock(box());
                         block.scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);
                         block.layoutBlock(true);
                     } else {
@@ skipping 889 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink