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 04622787915d680a92a983f62357410b1f12ba7a..28e4e8e636dff8fb4236cd9f8026fe3365b7b17b 100644
--- a/Source/core/css/resolver/CSSToStyleMap.cpp
+++ b/Source/core/css/resolver/CSSToStyleMap.cpp
@@ -479,6 +479,9 @@ 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;
@@ -493,7 +496,7 @@ 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 nullptr;
@@ -502,8 +505,15 @@ 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) {
+        // Step middle timing functions are supported up to this point for use in the Web Animations API,
+        // but should not be supported for CSS Animations and Transitions.
+        bool isStepMiddleFunction = (timingFunction->type() == TimingFunction::StepsFunction) && (toStepsTimingFunction(*timingFunction).stepAtPosition() == StepsTimingFunction::StepAtMiddle);
+        if (isStepMiddleFunction)
+            animation->setTimingFunction(CubicBezierTimingFunction::preset(CubicBezierTimingFunction::Ease));
+        else
+            animation->setTimingFunction(timingFunction);
+    }
 }
 
 void CSSToStyleMap::mapNinePieceImage(RenderStyle* mutableStyle, CSSPropertyID property, CSSValue* value, NinePieceImage& image)