Add composite() method to InterpolationType interface

Source/core/animation/UnderlyingValue.h

Index: Source/core/animation/UnderlyingValue.h
diff --git a/Source/core/animation/UnderlyingValue.h b/Source/core/animation/UnderlyingValue.h
new file mode 100644
index 0000000000000000000000000000000000000000..6d76d8fd866865e678c19e11ff99d5ccd5d05c00
--- /dev/null
+++ b/Source/core/animation/UnderlyingValue.h
@@ -0,0 +1,59 @@
+// Copyright 2015 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 UnderlyingValue_h
+#define UnderlyingValue_h
+
+#include "core/animation/InterpolationValue.h"
+#include "wtf/Allocator.h"
+
+namespace blink {
+
+// Handles memory management of underlying InterpolationValues in applyStack()
+// Ensures we perform copy on write if we are not the owner of an underlying InterpolationValue.
+// This functions similar to a DataRef except on OwnPtr'd objects.
+class UnderlyingValue {
+    STACK_ALLOCATED();
+
+public:
+    UnderlyingValue()
+        : m_valueOwner(nullptr)
+        , m_value(nullptr)
+    { }
+
+    void set(const InterpolationValue* interpolationValue)
+    {
+        // By clearing m_valueOwner we will perform a copy before attempting to mutate m_value,
+        // thus upholding the const contract for this instance of interpolationValue despite the const_cast.
+        m_valueOwner.clear();
+        m_value = const_cast<InterpolationValue*>(interpolationValue);
+    }
+    void set(PassOwnPtr<InterpolationValue> interpolationValue)
+    {
+        m_valueOwner = interpolationValue;
+        m_value = m_valueOwner.get();
+    }
+    InterpolationComponentValue& mutableComponent()
+    {
+        ASSERT(m_value);
+        if (!m_valueOwner)
+            set(m_value->clone());
+        return m_value->mutableComponent();
+    }
+    const InterpolationValue* get() const { return m_value; }
+    operator bool() const { return m_value; }
+    const InterpolationValue* operator->() const
+    {
+        ASSERT(m_value);
+        return m_value;
+    }
+
+private:
+    OwnPtr<InterpolationValue> m_valueOwner;
+    InterpolationValue* m_value;
+};
+
+} // namespace blink
+
+#endif // UnderlyingValue_h