Revert r160071 "Web Animations: Implement AnimationClock and fix start time of animations"

Source/core/animation/DocumentTimeline.h

Index: Source/core/animation/DocumentTimeline.h
diff --git a/Source/core/animation/DocumentTimeline.h b/Source/core/animation/DocumentTimeline.h
index 1cddc56fec79b5e2a4c493c3c6551973ddbf01a9..9323f55d156489e6ad606bf2f375a79cda6c725d 100644
--- a/Source/core/animation/DocumentTimeline.h
+++ b/Source/core/animation/DocumentTimeline.h
@@ -54,7 +54,7 @@ public:
     // Called from setReadyState() in Document.cpp to set m_zeroTimeAsPerfTime to
     // performance.timing.domInteractive.
     void setZeroTimeAsPerfTime(double);
-    double currentTime();
+    double currentTime() { return m_currentTime; }
     void pauseAnimationsForTesting(double);
     size_t numberOfActiveAnimationsForTesting() const;
     AnimationStack* animationStack(const Element* element) const
@@ -71,6 +71,7 @@ public:
 private:
     DocumentTimeline(Document*);
     void dispatchEvents();
+    double m_currentTime;
     double m_zeroTimeAsPerfTime;
     Document* m_document;
     Vector<RefPtr<Player> > m_players;