Animations: Allow document animation timelines to have a playback rate

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 52650095c14df569cc17fceac1dabafa537f00c3..1f1abbbb2f7e6f1563617f1630f4146d04f1905e 100644
--- a/Source/core/animation/AnimationTimeline.h
+++ b/Source/core/animation/AnimationTimeline.h
@@ -91,6 +91,9 @@ public:
     void setOutdatedAnimationPlayer(AnimationPlayer*);
     bool hasOutdatedAnimationPlayer() const;
 
+    void setPlaybackRate(double);
+    double playbackRate() const;
+
     Document* document() { return m_document.get(); }
 #if !ENABLE(OILPAN)
     void detachFromDocument();
@@ -110,6 +113,10 @@ private:
     WillBeHeapHashSet<RefPtrWillBeMember<AnimationPlayer>> m_playersNeedingUpdate;
     WillBeHeapHashSet<RawPtrWillBeWeakMember<AnimationPlayer>> m_players;
 
+    double m_currentTimeSnapshot;
+    double m_rawCurrentTimeSnapshot;
+    double m_playbackRate;
+
     friend class SMILTimeContainer;
     static const double s_minimumDelay;