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

Source/core/css/resolver/CSSToStyleMap.cpp

Index: Source/core/css/resolver/CSSToStyleMap.cpp
diff --git a/Source/core/css/resolver/CSSToStyleMap.cpp b/Source/core/css/resolver/CSSToStyleMap.cpp
index 406b16c3d5bf155e03b5a1fc63cd26f9a542f4fc..c8c2a435c98030056faaaecb3b45d2116b7d27b5 100644
--- a/Source/core/css/resolver/CSSToStyleMap.cpp
+++ b/Source/core/css/resolver/CSSToStyleMap.cpp
@@ -475,6 +475,9 @@ PassRefPtr<TimingFunction> CSSToStyleMap::animationTimingFunction(CSSValue* valu
         case CSSValueStepStart:
             return StepsTimingFunction::preset(StepsTimingFunction::Start);
             break;
+        case CSSValueStepMiddle:
+            return StepsTimingFunction::preset(StepsTimingFunction::Middle);
+            break;
         case CSSValueStepEnd:
             return StepsTimingFunction::preset(StepsTimingFunction::End);
             break;
@@ -489,7 +492,7 @@ PassRefPtr<TimingFunction> CSSToStyleMap::animationTimingFunction(CSSValue* valu
         return CubicBezierTimingFunction::create(cubicTimingFunction->x1(), cubicTimingFunction->y1(), cubicTimingFunction->x2(), cubicTimingFunction->y2());
     } else if (value->isStepsTimingFunctionValue()) {
         CSSStepsTimingFunctionValue* stepsTimingFunction = toCSSStepsTimingFunctionValue(value);
-        return StepsTimingFunction::create(stepsTimingFunction->numberOfSteps(), stepsTimingFunction->stepAtStart());
+        return StepsTimingFunction::create(stepsTimingFunction->numberOfSteps(), stepsTimingFunction->stepAtPosition());
     }
 
     return 0;
@@ -498,8 +501,11 @@ PassRefPtr<TimingFunction> CSSToStyleMap::animationTimingFunction(CSSValue* valu
 void CSSToStyleMap::mapAnimationTimingFunction(CSSAnimationData* animation, CSSValue* value) const
 {
     RefPtr<TimingFunction> timingFunction = animationTimingFunction(value, true);
-    if (timingFunction)
-        animation->setTimingFunction(timingFunction);
+    if (timingFunction) {
+        bool isStepMiddleFunction = (timingFunction->type() == TimingFunction::StepsFunction) && (toStepsTimingFunction(*timingFunction).stepAtPosition() == StepsTimingFunction::StepAtMiddle);
+        if (!isStepMiddleFunction)
+            animation->setTimingFunction(timingFunction);
+    }
 }
 
 void CSSToStyleMap::mapNinePieceImage(RenderStyle* mutableStyle, CSSPropertyID property, CSSValue* value, NinePieceImage& image)