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 a842af0fcbb6a0ecf446bcdc6c2d0dd0db17dadf..beca3db63cd73ed5d60c5cdc4b1adb37b2dc94ae 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -686,6 +686,8 @@ 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 {