Add additive animation support for CSS property background-position

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

Index: third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.cpp
diff --git a/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.cpp b/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..93d79823aaaa3af4b6158a7f75e8120fd7e84fad
--- /dev/null
+++ b/third_party/WebKit/Source/core/animation/CSSPositionAxisListInterpolationType.cpp
@@ -0,0 +1,42 @@
+// 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.
+
+#include "core/animation/CSSPositionAxisListInterpolationType.h"
+
+#include "core/animation/CSSLengthInterpolationType.h"
+#include "core/animation/ListInterpolationFunctions.h"
+#include "core/css/CSSPrimitiveValue.h"
+#include "core/css/CSSValueList.h"
+#include "core/css/CSSValuePair.h"
+
+namespace blink {
+
+static InterpolationValue convertPositionAxisCSSValue(const CSSValue& value)
+{
+    if (!value.isValuePair())
+        return CSSLengthInterpolationType::maybeConvertCSSValue(value);
+
+    const CSSValuePair& pair = toCSSValuePair(value);
+    InterpolationValue result = CSSLengthInterpolationType::maybeConvertCSSValue(pair.second());
+    CSSValueID side = toCSSPrimitiveValue(pair.first()).getValueID();
+    if (side == CSSValueRight || side == CSSValueBottom)
+        CSSLengthInterpolationType::subtractFromOneHundredPercent(result);
+    return result;
+}
+
+InterpolationValue CSSPositionAxisListInterpolationType::maybeConvertValue(const CSSValue& value, const StyleResolverState&, ConversionCheckers&) const
+{
+    if (!value.isBaseValueList()) {
+        return ListInterpolationFunctions::createList(1, [&value](size_t) {
+            return convertPositionAxisCSSValue(value);
+        });
+    }
+
+    const CSSValueList& list = toCSSValueList(value);
+    return ListInterpolationFunctions::createList(list.length(), [&list](size_t index) {
+        return convertPositionAxisCSSValue(*list.item(index));
+    });
+}
+
+} // namespace blink