Implemented additive animations for length

Source/core/animation/InterpolationFactory.h

Index: Source/core/animation/InterpolationFactory.h
diff --git a/Source/core/animation/InterpolationFactory.h b/Source/core/animation/InterpolationFactory.h
new file mode 100644
index 0000000000000000000000000000000000000000..f0d4c975d3f056910230c7e1ebc36f1a71d4676c
--- /dev/null
+++ b/Source/core/animation/InterpolationFactory.h
@@ -0,0 +1,38 @@
+// 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 InterpolationFactory_h
+#define InterpolationFactory_h
+
+#include "core/animation/AnimationEffect.h"
+#include "core/animation/Interpolation.h"
+#include "core/dom/Element.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class InterpolationFactory {
+public:
+
+    // FIXME: Remove use of animatableValueCache and element
+    static PassRefPtrWillBeRawPtr<Interpolation> maybeCreateInterpolation(
+        CSSPropertyID,
+        CSSValue* fromCSSValue,
+        CSSValue* toCSSValue,
+        AnimationEffect::CompositeOperation fromComposite,
+        AnimationEffect::CompositeOperation toComposite,
+        RefPtrWillBeRawPtr<AnimatableValue>* fromAnimatableValueCache = nullptr,
+        RefPtrWillBeRawPtr<AnimatableValue>* toAnimatableValueCache = nullptr,
+        Element* = nullptr);
+
+    static PassRefPtrWillBeRawPtr<Interpolation> createConstantInterpolation(
+        CSSPropertyID,
+        CSSValue*,
+        AnimationEffect::CompositeOperation fromComposite,
+        AnimationEffect::CompositeOperation toComposite);
+};
+
+} // namespace blink
+
+#endif // InterpolationFactory_h