Web Animations: Simplify mutable AnimationTimeline currentTime and playbackRate

Source/core/animation/AnimationTimeline.h

Index: Source/core/animation/AnimationTimeline.h
diff --git a/Source/core/animation/AnimationTimeline.h b/Source/core/animation/AnimationTimeline.h
index 1e826d6c3c4b5b4dccf9805aafb05e1b8c9bdfb7..faaef00ebd87d8f0faa26591c8c086f045cdf055 100644
--- a/Source/core/animation/AnimationTimeline.h
+++ b/Source/core/animation/AnimationTimeline.h
@@ -112,13 +112,12 @@ protected:
 private:
     RawPtrWillBeMember<Document> m_document;
     double m_zeroTime;
+    bool m_zeroTimeInitialized;
     // AnimationPlayers which will be updated on the next frame
     // i.e. current, in effect, or had timing changed
     WillBeHeapHashSet<RefPtrWillBeMember<AnimationPlayer>> m_playersNeedingUpdate;
     WillBeHeapHashSet<RawPtrWillBeWeakMember<AnimationPlayer>> m_players;
 
-    double m_documentCurrentTimeSnapshot;
-    double m_zeroTimeOffset;
     double m_playbackRate;
 
     friend class SMILTimeContainer;