Set m_{width,height}AvailableToChildrenChanged when scrollbars appear

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 656b4effafa988901318f24c2df30272ccbf154b..5f441befd71e298a10e2d1941b88b298653d3530 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -681,6 +681,8 @@ bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayo
             if (!m_inOverflowRelayout) {
                 m_inOverflowRelayout = true;
                 if (delayedLayoutScope) {
+                    if (box().isLayoutBlock())
+                        toLayoutBlock(box()).scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);
                     delayedLayoutScope->setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
                     didMarkForDelayedLayout = true;
                 } else {