Allow layers that need squashing to draw into the squashing layer in some cases.

Source/core/rendering/compositing/CompositedLayerMappingPtr.h

Index: Source/core/rendering/compositing/CompositedLayerMappingPtr.h
diff --git a/Source/core/rendering/compositing/CompositedLayerMappingPtr.h b/Source/core/rendering/compositing/CompositedLayerMappingPtr.h
index 27cb68581d5ff74c2c7c4fda540350bbb71a19c6..9f2825c35c2ef1d555e1a407cd283bf6b762c243 100644
--- a/Source/core/rendering/compositing/CompositedLayerMappingPtr.h
+++ b/Source/core/rendering/compositing/CompositedLayerMappingPtr.h
@@ -30,6 +30,11 @@ public:
         return m_mapping;
     }
 
+    bool equals(const CompositedLayerMapping* other) const
+    {
+        return m_mapping == other;
+    }
+
 private:
     CompositedLayerMapping* m_mapping;
 };