Web Animations API: Implement step-middle and steps(x, middle) timing functions.

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 2b941b5d8f22f06f4b2ef326f4f26775ab28f913..f15c4ee9fbd7e5a779234acd289cf7822f8e22d0 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -1021,13 +1021,19 @@ static PassRefPtr<CSSValue> createTimingFunctionValue(const TimingFunction* timi
     case TimingFunction::StepsFunction:
         {
             const StepsTimingFunction* stepsTimingFunction = toStepsTimingFunction(timingFunction);
-            if (stepsTimingFunction->subType() == StepsTimingFunction::Custom)
-                return CSSStepsTimingFunctionValue::create(stepsTimingFunction->numberOfSteps(), stepsTimingFunction->stepAtStart());
+            if (stepsTimingFunction->subType() == StepsTimingFunction::Custom) {
+                if (stepsTimingFunction->stepAtPosition() != StepsTimingFunction::StepAtMiddle)
+                    return CSSStepsTimingFunctionValue::create(stepsTimingFunction->numberOfSteps(), stepsTimingFunction->stepAtPosition());
+                return cssValuePool().createIdentifierValue(CSSValueLinear);
+            }
             CSSValueID valueId;
             switch (stepsTimingFunction->subType()) {
             case StepsTimingFunction::Start:
                 valueId = CSSValueStepStart;
                 break;
+            case StepsTimingFunction::Middle:
+                valueId = CSSValueLinear;
+                break;
             case StepsTimingFunction::End:
                 valueId = CSSValueStepEnd;
                 break;