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

Source/core/animation/css/CSSAnimations.cpp

Index: Source/core/animation/css/CSSAnimations.cpp
diff --git a/Source/core/animation/css/CSSAnimations.cpp b/Source/core/animation/css/CSSAnimations.cpp
index 281fea9593df8aa436fe62b624b4f0408382af41..fbb97666d62820dc113137fce5b2a9d9d2561874 100644
--- a/Source/core/animation/css/CSSAnimations.cpp
+++ b/Source/core/animation/css/CSSAnimations.cpp
@@ -136,8 +136,10 @@ PassOwnPtr<CSSAnimationUpdate> CSSAnimations::calculateUpdate(const Element* ele
             }
 
             // 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) {