Add additive animation support for CSS property background-position

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

Index: third_party/WebKit/Source/core/animation/CSSLengthInterpolationType.cpp
diff --git a/third_party/WebKit/Source/core/animation/CSSLengthInterpolationType.cpp b/third_party/WebKit/Source/core/animation/CSSLengthInterpolationType.cpp
index 8a97b542f2130403c2d5c14bfad8256cadeecffe..46b17a8c73c549cb68dee2155347710594bf2d1c 100644
--- a/third_party/WebKit/Source/core/animation/CSSLengthInterpolationType.cpp
+++ b/third_party/WebKit/Source/core/animation/CSSLengthInterpolationType.cpp
@@ -105,6 +105,18 @@ void CSSLengthInterpolationType::composite(
     underlyingNonInterpolableValue = CSSLengthNonInterpolableValue::merge(underlyingNonInterpolableValue.get(), nonInterpolableValue);
 }
 
+void CSSLengthInterpolationType::subtractFromOneHundredPercent(InterpolationValue& result)
+{
+    InterpolableList& list = toInterpolableList(*result.interpolableValue);
+    for (size_t i = 0; i < CSSPrimitiveValue::LengthUnitTypeCount; i++) {
+        double value = -toInterpolableNumber(*list.get(i)).value();
+        if (i == CSSPrimitiveValue::UnitTypePercentage)
+            value += 100;
+        toInterpolableNumber(*list.getMutable(i)).set(value);
+    }
+    result.nonInterpolableValue = CSSLengthNonInterpolableValue::create(true);
+}
+
 InterpolationValue CSSLengthInterpolationType::maybeConvertCSSValue(const CSSValue& value)
 {
     if (!value.isPrimitiveValue())