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..0c34a1534baa6187a68c75ee55a96abf9260d2dd 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -681,6 +681,10 @@ bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayo
             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 {