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..aebf0d1cbe07a85fa2f208a83e6ad97b7f60272a 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -682,6 +682,10 @@ bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayo
                 m_inOverflowRelayout = true;
                 if (delayedLayoutScope) {
                     delayedLayoutScope->setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
+                    // Since we can't pass true to layoutBlock to force relaying out the children, mark them explicitly.
+                    // They do have to be laid out again because their available width has now changed.
+                    for (LayoutObject* child = box().slowFirstChild(); child; child = child->nextSibling())
+                        delayedLayoutScope->setNeedsLayout(child, LayoutInvalidationReason::ScrollbarChanged);
                     didMarkForDelayedLayout = true;
                 } else {
                     SubtreeLayoutScope layoutScope(box());