Web Animations CSS: Assert underlying value is not used

Source/core/css/resolver/StyleResolver.cpp

Index: Source/core/css/resolver/StyleResolver.cpp
diff --git a/Source/core/css/resolver/StyleResolver.cpp b/Source/core/css/resolver/StyleResolver.cpp
index 996285ae47ae034b75e0ec582de59633b31ed04c..d8e32420200e9086117f0f89be5e3eee6af0eeea 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -1162,7 +1162,8 @@ void StyleResolver::applyAnimatedProperties(StyleResolverState& state, const Ele
             CSSPropertyID property = iter->key;
             if (!isPropertyForPass<pass>(property))
                 continue;
-            RefPtr<AnimatableValue> animatableValue = iter->value->compositeOnto(AnimatableValue::neutralValue());
+            RELEASE_ASSERT_WITH_MESSAGE(!iter->value->dependsOnUnderlyingValue(), "Not yet implemented: An interface for compositing onto the underlying value.");
+            RefPtr<AnimatableValue> animatableValue = iter->value->compositeOnto(0);
             if (pass == HighPriorityProperties && property == CSSPropertyLineHeight)
                 state.setLineHeightValue(toAnimatableNumber(animatableValue.get())->toCSSValue().get());
             else