Web Animations: Address some review feedback to clean up CSS Animations

Source/core/animation/css/CSSAnimations.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 118 matching lines @@
 
             if (cssAnimations) {
                 AnimationMap::const_iterator existing(cssAnimations->m_animations.find(animationName.impl()));
                 if (existing != cssAnimations->m_animations.end()) {
                     inactive.remove(animationName.impl());
                     continue;
                 }
             }
 
             // If there's a delay, no styles will apply yet.
-            if (animationData->isDelaySet() && animationData->delay())
+            if (animationData->isDelaySet() && animationData->delay()) {
+                RELEASE_ASSERT_WITH_MESSAGE(animationData->delay() > 0, "Negative delay is not yet supported.");
                 continue;
+            }
 
             const StylePropertySet* keyframeStyles = resolver->firstKeyframeStyles(element, animationName.impl());
             if (keyframeStyles) {
                 if (!update)
                     update = adoptPtr(new CSSAnimationUpdate());
                 update->addStyles(keyframeStyles);
             }
         }
     }
 
@@ skipping 43 matching lines @@
 
 void CSSAnimations::cancel()
 {
     for (AnimationMap::iterator iter = m_animations.begin(); iter != m_animations.end(); ++iter)
         iter->value->cancel();
 
     m_animations.clear();
 }
 
 } // namespace WebCore