Use a boolean hasCompositedLayerMapping() accessor instead of the pointer

Source/core/rendering/RenderLayerModelObject.h

Index: Source/core/rendering/RenderLayerModelObject.h
diff --git a/Source/core/rendering/RenderLayerModelObject.h b/Source/core/rendering/RenderLayerModelObject.h
index ae7d74e4119b87cf6716e11daedc3204b2e9d0b5..819e4838f4193acd8dddb8e11f9de50dadb3f4d6 100644
--- a/Source/core/rendering/RenderLayerModelObject.h
+++ b/Source/core/rendering/RenderLayerModelObject.h
@@ -23,6 +23,7 @@
 #ifndef RenderLayerModelObject_h
 #define RenderLayerModelObject_h
 
+#include "core/rendering/CompositedLayerMappingPtr.h"
 #include "core/rendering/RenderObject.h"
 
 namespace WebCore {
@@ -56,7 +57,8 @@ public:
     // This is null for anonymous renderers.
     ContainerNode* node() const { return toContainerNode(RenderObject::node()); }
 
-    CompositedLayerMapping* compositedLayerMapping() const;
+    CompositedLayerMappingPtr compositedLayerMapping() const;
+    bool hasCompositedLayerMapping() const;
 
 protected:
     void createLayer();