Refactor how UseCounter::SyntheticKeyframesInCompositedCSSAnimation is counted

third_party/WebKit/Source/core/animation/Keyframe.h

Index: third_party/WebKit/Source/core/animation/Keyframe.h
diff --git a/third_party/WebKit/Source/core/animation/Keyframe.h b/third_party/WebKit/Source/core/animation/Keyframe.h
index 599e362893d089320f30e53ad65edb5985c17a23..bbafe935a027b6997d5157179cf56bae6c898f86 100644
--- a/third_party/WebKit/Source/core/animation/Keyframe.h
+++ b/third_party/WebKit/Source/core/animation/Keyframe.h
@@ -73,10 +73,7 @@ class CORE_EXPORT Keyframe : public RefCounted<Keyframe> {
     double underlyingFraction() const {
       return m_composite == EffectModel::CompositeReplace ? 0 : 1;
     }
-    virtual bool isNeutral() const {
-      NOTREACHED();
-      return false;
-    }
+    virtual bool isNeutral() const = 0;
     virtual PassRefPtr<PropertySpecificKeyframe> cloneWithOffset(
         double offset) const = 0;