Web Animations: Simplify mutable AnimationTimeline currentTime and playbackRate

Source/core/animation/AnimationTimeline.h

 /*
  * 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 94 matching lines @@
     void wake();
 
     void trace(Visitor*);
 
 protected:
     AnimationTimeline(Document*, PassOwnPtrWillBeRawPtr<PlatformTiming>);
 
 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;
     static const double s_minimumDelay;
 
     OwnPtrWillBeMember<PlatformTiming> m_timing;
     double m_lastCurrentTimeInternal;
 
     class AnimationTimelineTiming final : public PlatformTiming {
     public:
@@ skipping 16 matching lines @@
         RawPtrWillBeMember<AnimationTimeline> m_timeline;
         Timer<AnimationTimelineTiming> m_timer;
     };
 
     friend class AnimationAnimationTimelineTest;
 };
 
 } // namespace blink
 
 #endif