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

Source/platform/animation/TimingFunction.cpp

Index: Source/platform/animation/TimingFunction.cpp
diff --git a/Source/platform/animation/TimingFunction.cpp b/Source/platform/animation/TimingFunction.cpp
index f196dab07a1270fabd7cc1599bd541351438e967..adde4002bb4560dfe76e0af741e5414510995939 100644
--- a/Source/platform/animation/TimingFunction.cpp
+++ b/Source/platform/animation/TimingFunction.cpp
@@ -47,17 +47,29 @@ double CubicBezierTimingFunction::evaluate(double fraction, double accuracy) con
     return m_bezier->solve(fraction, accuracy);
 }
 
+// FIXME: Unit test this thoroughly
 String StepsTimingFunction::toString() const
 {
     StringBuilder builder;
     switch (this->subType()) {
     case StepsTimingFunction::Start:
         return "step-start";
+    case StepsTimingFunction::Middle:
+        return "step-middle";
     case StepsTimingFunction::End:
         return "step-end";
     case StepsTimingFunction::Custom:
         builder.append("steps(" + String::numberToStringECMAScript(this->numberOfSteps()) + ", ");
-        builder.append(this->stepAtStart() ? "start" : "end");
+
+        if (this->stepAtPosition() == StepsTimingFunction::StepAtStart)
+            builder.append("start");
+        else if (this->stepAtPosition() == StepsTimingFunction::StepAtMiddle)
+            builder.append("middle");
+        else if (this->stepAtPosition() == StepsTimingFunction::StepAtEnd)
+            builder.append("end");
+        else
+            ASSERT_NOT_REACHED();
+
         builder.append(")");
         break;
     default:
@@ -69,7 +81,22 @@ String StepsTimingFunction::toString() const
 double StepsTimingFunction::evaluate(double fraction, double) const
 {
     ASSERT_WITH_MESSAGE(fraction >= 0 && fraction <= 1, "Web Animations not yet implemented: Timing function behavior outside the range [0, 1] is not yet specified");
-    return std::min(1.0, (floor(m_steps * fraction) + m_stepAtStart) / m_steps);
+    double startOffset = 0;
+    switch (m_stepAtPosition) {
+    case StepAtStart:
+        startOffset = 1;
+        break;
+    case StepAtMiddle:
+        startOffset = 0.5;
+        break;
+    case StepAtEnd:
+        startOffset = 0;
+        break;
+    default:
+        ASSERT_NOT_REACHED();
+        break;
+    }
+    return std::min(1.0, floor((m_steps * fraction) + startOffset) / m_steps);
 }
 
 String ChainedTimingFunction::toString() const