Add additive animation support for CSS property background-position

third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.h

Index: third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.h
diff --git a/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.h b/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.h
new file mode 100644
index 0000000000000000000000000000000000000000..b42b131781cbdde6dfb9fdc8b875a375afce658b
--- /dev/null
+++ b/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.h
@@ -0,0 +1,24 @@
+// Copyright 2016 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 CSSPositionAxisListInterpolationType_h
+#define CSSPositionAxisListInterpolationType_h
+
+#include "core/animation/CSSLengthListInterpolationType.h"
+
+namespace blink {
+
+class CSSPositionAxisListInterpolationType : public CSSLengthListInterpolationType {
+public:
+    CSSPositionAxisListInterpolationType(CSSPropertyID property)
+        : CSSLengthListInterpolationType(property)
+    { }
+
+private:
+    InterpolationValue maybeConvertValue(const CSSValue&, const StyleResolverState&, ConversionCheckers&) const final;
+};
+
+} // namespace blink
+
+#endif // CSSPositionAxisListInterpolationType_h