Add use counter for stacked neutral keyframe in 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 48c7f5cd54caf7f8c7b706297278be2066d5daa3..493ae473deb86d8f27baadb9b1bd3d2361ffcc19 100644
--- a/Source/core/animation/css/CSSAnimations.cpp
+++ b/Source/core/animation/css/CSSAnimations.cpp
@@ -65,7 +65,7 @@ using PropertySet = HashSet<CSSPropertyID>;
 
 namespace {
 
-static PassRefPtrWillBeRawPtr<StringKeyframeEffectModel> createKeyframeEffectModel(StyleResolver* resolver, const Element* animatingElement, Element& element, const ComputedStyle* style, const ComputedStyle* parentStyle, const AtomicString& name, TimingFunction* defaultTimingFunction)
+static PassRefPtrWillBeRawPtr<StringKeyframeEffectModel> createKeyframeEffectModel(StyleResolver* resolver, const Element* animatingElement, Element& element, const ComputedStyle* style, const ComputedStyle* parentStyle, const AtomicString& name, TimingFunction* defaultTimingFunction, size_t animationIndex)
 {
     // When the animating element is null, use its parent for scoping purposes.
     const Element* elementForScoping = animatingElement ? animatingElement : &element;
@@ -173,6 +173,8 @@ static PassRefPtrWillBeRawPtr<StringKeyframeEffectModel> createKeyframeEffectMod
 
     RefPtrWillBeRawPtr<StringKeyframeEffectModel> model = StringKeyframeEffectModel::create(keyframes, &keyframes[0]->easing());
     model->forceConversionsToAnimatableValues(element, style);
+    if (animationIndex > 0 && model->hasSyntheticKeyframes())
+        UseCounter::count(elementForScoping->document(), UseCounter::CSSAnimationsStackedNeutralKeyframe);
     return model;
 }
 
@@ -261,7 +263,7 @@ void CSSAnimations::calculateAnimationUpdate(CSSAnimationUpdate* update, const E
                     if (keyframesRule != runningAnimation->styleRule || keyframesRule->version() != runningAnimation->styleRuleVersion || runningAnimation->specifiedTiming != specifiedTiming) {
                         ASSERT(!isAnimationStyleChange);
                         update->updateAnimation(animationName, animation, InertEffect::create(
-                            createKeyframeEffectModel(resolver, animatingElement, element, &style, parentStyle, animationName, keyframeTimingFunction.get()),
+                            createKeyframeEffectModel(resolver, animatingElement, element, &style, parentStyle, animationName, keyframeTimingFunction.get(), i),
                             timing, isPaused, animation->unlimitedCurrentTimeInternal()), specifiedTiming, keyframesRule);
                     } else if (!isAnimationStyleChange && animation->effect() && animation->effect()->isAnimation()) {
                         EffectModel* model = toKeyframeEffect(animation->effect())->model();
@@ -283,7 +285,7 @@ void CSSAnimations::calculateAnimationUpdate(CSSAnimationUpdate* update, const E
 
             ASSERT(!isAnimationStyleChange);
             update->startAnimation(animationName, InertEffect::create(
-                createKeyframeEffectModel(resolver, animatingElement, element, &style, parentStyle, animationName, keyframeTimingFunction.get()),
+                createKeyframeEffectModel(resolver, animatingElement, element, &style, parentStyle, animationName, keyframeTimingFunction.get(), i),
                 timing, isPaused, 0), specifiedTiming, keyframesRule);
         }
     }