Simplify animation/transition parsing slightly

Source/core/css/parser/BisonCSSParser-in.cpp

Index: Source/core/css/parser/BisonCSSParser-in.cpp
diff --git a/Source/core/css/parser/BisonCSSParser-in.cpp b/Source/core/css/parser/BisonCSSParser-in.cpp
index add82fdaf1d377746676bde932d0c69ba0c95e40..7fe8e93b65ae7360fe1dff48559cd92c1f12ab12 100644
--- a/Source/core/css/parser/BisonCSSParser-in.cpp
+++ b/Source/core/css/parser/BisonCSSParser-in.cpp
@@ -921,7 +921,13 @@ PassRefPtrWillBeRawPtr<CSSValue> BisonCSSParser::parseAnimationTimingFunctionVal
     if (!parseValue(style.get(), CSSPropertyTransitionTimingFunction, string, false, HTMLStandardMode, 0))
         return nullptr;
 
-    return style->getPropertyCSSValue(CSSPropertyTransitionTimingFunction);
+    RefPtrWillBeRawPtr<CSSValue> value = style->getPropertyCSSValue(CSSPropertyTransitionTimingFunction);
+    if (!value || value->isInitialValue() || value->isInheritedValue())
+        return nullptr;
+    CSSValueList* valueList = toCSSValueList(value.get());
+    if (valueList->length() > 1)
+        return nullptr;
+    return valueList->item(0);
 }
 
 bool BisonCSSParser::parseValue(MutableStylePropertySet* declaration, CSSPropertyID propertyID, const String& string, bool important, const Document& document)