Web Animations: Zero time of the document timeline should correspond with navigationStart

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 62 matching lines @@
 
 #if !ENABLE(OILPAN)
     void playerDestroyed(AnimationPlayer* player)
     {
         ASSERT(m_players.contains(player));
         m_players.remove(player);
     }
 #endif
 
     bool hasPendingUpdates() const { return !m_playersNeedingUpdate.isEmpty(); }
-    double zeroTime() const { return 0; }
+    double zeroTime();
     double currentTime(bool& isNull);
     double currentTime();
     double currentTimeInternal(bool& isNull);
     double currentTimeInternal();
     double effectiveTime();
     void pauseAnimationsForTesting(double);
 
     void setOutdatedAnimationPlayer(AnimationPlayer*);
     bool hasOutdatedAnimationPlayer() const;
 
     Document* document() { return m_document.get(); }
 #if !ENABLE(OILPAN)
     void detachFromDocument();
 #endif
     void wake();
 
     void trace(Visitor*);
 
 protected:
     AnimationTimeline(Document*, PassOwnPtrWillBeRawPtr<PlatformTiming>);
 
 private:
     RawPtrWillBeMember<Document> m_document;
+    double m_zeroTime;
     // 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;
 
     friend class SMILTimeContainer;
     static const double s_minimumDelay;
 
     OwnPtrWillBeMember<PlatformTiming> m_timing;
 
@@ skipping 18 matching lines @@
         RawPtrWillBeMember<AnimationTimeline> m_timeline;
         Timer<AnimationTimelineTiming> m_timer;
     };
 
     friend class AnimationAnimationTimelineTest;
 };
 
 } // namespace blink
 
 #endif