Remove background obscuration checks.

sky/engine/core/rendering/RenderLayerModelObject.h

Index: sky/engine/core/rendering/RenderLayerModelObject.h
diff --git a/sky/engine/core/rendering/RenderLayerModelObject.h b/sky/engine/core/rendering/RenderLayerModelObject.h
index 30969422c9a4ab9b71b3b1eb7b7bae5d7e565dc8..d802d71b62bcee1b6b1091c08a63f9a506b212a0 100644
--- a/sky/engine/core/rendering/RenderLayerModelObject.h
+++ b/sky/engine/core/rendering/RenderLayerModelObject.h
@@ -57,10 +57,6 @@ public:
 
     virtual LayerType layerTypeRequired() const = 0;
 
-    // Returns true if the background is painted opaque in the given rect.
-    // The query rect is given in local coordinate system.
-    virtual bool backgroundIsKnownToBeOpaqueInRect(const LayoutRect&) const { return false; }
-
     // This is null for anonymous renderers.
     ContainerNode* node() const { return toContainerNode(RenderObject::node()); }
 protected: