Use a boolean hasCompositedLayerMapping() accessor instead of the pointer

Source/core/rendering/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 {
+
+class CompositedLayerMapping;
+
+class CompositedLayerMappingPtr {
+public:
+    CompositedLayerMappingPtr(CompositedLayerMapping* mapping)
+        : m_mapping(mapping)
+    {
+    }
+
+    CompositedLayerMapping& operator*() const
+    {
+        ASSERT(m_mapping);
+        return *m_mapping;
+    }
+
+    CompositedLayerMapping* operator->() const
+    {
+        ASSERT(m_mapping);
+        return m_mapping;
+    }
+
+private:
+    CompositedLayerMapping* m_mapping;
+};
+
+} // namespace WebCore
+
+#endif // CompositedLayerMappingPtr_h