Add foundation for removing AnimatableValues from StyleInterpolation

Source/core/animation/NonInterpolableValue.h

+// 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 NonInterpolableValue_h
+#define NonInterpolableValue_h
+
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+// Holds components of a keyframe value that do not change when interpolating with another keyframe.
+class NonInterpolableValue : public RefCountedWillBeGarbageCollectedFinalized<NonInterpolableValue> {
+public:
+    virtual ~NonInterpolableValue() { }
+
+    typedef const void* Type;
+    virtual Type type() const = 0;
+
+    DEFINE_INLINE_VIRTUAL_TRACE() { }
+};
+
+// These macros provide safe downcasts of NonInterpolableValue subclasses with debug assertions.
+// See CSSValueAnimationType for example usage.
+#define DECLARE_NON_INTERPOLABLE_VALUE_TYPE() \
+    static Type staticType; \
+    virtual Type type() const { return staticType; }
+
+#define DEFINE_NON_INTERPOLABLE_VALUE_TYPE(T) \
+    NonInterpolableValue::Type T::staticType = &T::staticType;
+
+#define DEFINE_NON_INTERPOLABLE_VALUE_TYPE_CASTS(T) \
+    DEFINE_TYPE_CASTS(T, NonInterpolableValue, value, value->type() == T::staticType, value.type() == T::staticType);
+
+} // namespace blink
+
+#endif // NonInterpolableValue_h