Merge 200866 "Do not create scrollbars if the subtree is invisible."

Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp

Index: Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp
===================================================================
--- Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp	(revision 201065)
+++ Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp	(working copy)
@@ -1306,6 +1306,12 @@
 
 bool CompositedDeprecatedPaintLayerMapping::updateOverflowControlsLayers(bool needsHorizontalScrollbarLayer, bool needsVerticalScrollbarLayer, bool needsScrollCornerLayer, bool needsAncestorClip)
 {
+    // If the subtree is invisible, we don't actually need scrollbar layers.
+    bool invisible = m_owningLayer.subtreeIsInvisible();
+    needsHorizontalScrollbarLayer &= !invisible;
+    needsVerticalScrollbarLayer &= !invisible;
+    needsScrollCornerLayer &= !invisible;
+
     bool horizontalScrollbarLayerChanged = toggleScrollbarLayerIfNeeded(m_layerForHorizontalScrollbar, needsHorizontalScrollbarLayer, CompositingReasonLayerForHorizontalScrollbar);
     bool verticalScrollbarLayerChanged = toggleScrollbarLayerIfNeeded(m_layerForVerticalScrollbar, needsVerticalScrollbarLayer, CompositingReasonLayerForVerticalScrollbar);
     bool scrollCornerLayerChanged = toggleScrollbarLayerIfNeeded(m_layerForScrollCorner, needsScrollCornerLayer, CompositingReasonLayerForScrollCorner);