Only allocate a foreground layer for a CLM if there are composited negative-z-index-children.

Source/core/rendering/compositing/RenderLayerCompositor.cpp

Index: Source/core/rendering/compositing/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/compositing/RenderLayerCompositor.cpp b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
index 0d39968d532ac8a46fc5299e2f0ac6c0c2979914..36deffdd17e66262a0798ae4f2e10d324307505a 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -748,11 +748,13 @@ bool RenderLayerCompositor::clipsCompositingDescendants(const RenderLayer* layer
     return layer->hasCompositingDescendant() && layer->renderer()->hasClipOrOverflowClip();
 }
 
-// If an element has negative z-index children, those children render in front of the
+// If an element has composited negative z-index children, those children render in front of the
 // layer background, so we need an extra 'contents' layer for the foreground of the layer
 // object.
 bool RenderLayerCompositor::needsContentsCompositingLayer(const RenderLayer* layer) const
 {
+    if (!layer->hasCompositingDescendant())
+        return false;
     return layer->stackingNode()->hasNegativeZOrderList();
 }