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

Source/core/rendering/compositing/CompositedLayerMappingPtr.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositedLayerMappingPtr_h
 #define CompositedLayerMappingPtr_h
 
 #include "wtf/Assertions.h"
 
 namespace WebCore {
@@ skipping 12 matching lines @@
         ASSERT(m_mapping);
         return *m_mapping;
     }
 
     CompositedLayerMapping* operator->() const
     {
         ASSERT(m_mapping);
         return m_mapping;
     }
 
+    bool equals(const CompositedLayerMapping* other) const
+    {
+        return m_mapping == other;
+    }
+
 private:
     CompositedLayerMapping* m_mapping;
 };
 
 } // namespace WebCore
 
 #endif // CompositedLayerMappingPtr_h