Fix assertions related to floats and z-order lists when compositing opaque scrollers

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 52ad1408b3d2c41d93722417ea5935d6f6635614..e5977775a7632c0e4de6917883a52d3a08d9f2e4 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2366,10 +2366,11 @@ GraphicsLayer* PaintLayer::graphicsLayerBackingForScrolling() const
 
 bool PaintLayer::canPaintBackgroundOntoScrollingContentsLayer() const
 {
-    return !isRootLayer()
-        && scrollsOverflow()
-        && layoutObject()->hasLocalEquivalentBackground()
-        && !stackingNode()->hasNegativeZOrderList();
+    if (isRootLayer() || !scrollsOverflow() || !layoutObject()->hasLocalEquivalentBackground())
+        return false;
+
+    m_stackingNode->updateLayerListsIfNeeded();
+    return !m_stackingNode->hasNegativeZOrderList();
 }
 
 void PaintLayer::ensureCompositedLayerMapping()