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

Source/core/animation/ElementAnimation.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 102 matching lines @@
     if (direction == "reverse") {
         timing.direction = Timing::PlaybackDirectionReverse;
     } else if (direction == "alternate") {
         timing.direction = Timing::PlaybackDirectionAlternate;
     } else if (direction == "alternate-reverse") {
         timing.direction = Timing::PlaybackDirectionAlternateReverse;
     }
 
     String timingFunctionString;
     timingInputDictionary.get("easing", timingFunctionString);
-    RefPtr<CSSValue> timingFunctionValue = BisonCSSParser::parseAnimationTimingFunctionValue(timingFunctionString);
+    RefPtr<CSSValue> timingFunctionValue = BisonCSSParser::parseAnimationTimingFunctionValue(timingFunctionString, true);
     if (timingFunctionValue) {
         RefPtr<TimingFunction> timingFunction = CSSToStyleMap::animationTimingFunction(timingFunctionValue.get(), false);
         if (timingFunction)
             timing.timingFunction = timingFunction;
     }
 
     timing.assertValid();
 }
 
 static bool checkDocumentAndRenderer(Element* element)
@@ skipping 131 matching lines @@
 
     RefPtr<Animation> animation = Animation::create(element, effect, timing);
     DocumentTimeline* timeline = element->document().timeline();
     ASSERT(timeline);
     timeline->play(animation.get());
 
     return animation.get();
 }
 
 } // namespace WebCore