Add additive animation support for CSS property background-size

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

Index: third_party/WebKit/Source/core/animation/CSSSizeListInterpolationType.h
diff --git a/third_party/WebKit/Source/core/animation/CSSSizeListInterpolationType.h b/third_party/WebKit/Source/core/animation/CSSSizeListInterpolationType.h
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index 0000000000000000000000000000000000000000..5f9bf07a162469f960878ca262123cdf082661cb
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+++ b/third_party/WebKit/Source/core/animation/CSSSizeListInterpolationType.h
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+// 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 CSSSizeListInterpolationType_h
+#define CSSSizeListInterpolationType_h
+
+#include "core/animation/CSSLengthInterpolationType.h"
+#include "core/animation/CSSLengthListInterpolationType.h"
+#include "core/animation/ListInterpolationFunctions.h"
+#include "core/css/CSSValueList.h"
+#include "core/css/CSSValuePair.h"
+
+namespace blink {
+
+class CSSSizeListInterpolationType : public CSSLengthListInterpolationType {
+public:
+    CSSSizeListInterpolationType(CSSPropertyID property)
+        : CSSLengthListInterpolationType(property)
+    { }
+
+private:
+    InterpolationValue maybeConvertValue(const CSSValue& value, const StyleResolverState&, ConversionCheckers&) const final
+    {
+        RefPtrWillBeRawPtr<CSSValueList> tempList;
+        if (!value.isBaseValueList()) {
+            tempList = CSSValueList::createCommaSeparated();
+            tempList->append(const_cast<CSSValue*>(&value)); // Take ref.
+        }
+        const CSSValueList& list = value.isBaseValueList() ? toCSSValueList(value) : *tempList;
+
+        // Only size lists without keywords may interpolate smoothly:
+        // https://drafts.csswg.org/css-backgrounds-3/#the-background-size
+        return ListInterpolationFunctions::createList(list.length() * 2, [&list](size_t index) -> InterpolationValue {
+            const CSSValue& item = *list.item(index / 2);
+            if (!item.isValuePair())
+                return nullptr;
+            const CSSValuePair& pair = toCSSValuePair(item);
+            return CSSLengthInterpolationType::maybeConvertCSSValue(index % 2 == 0 ? pair.first() : pair.second());
+        });
+    }
+};
+
+} // namespace blink
+
+#endif // CSSSizeListInterpolationType_h