Ensure serviceOnNextFrame isn't put off indefinitely.

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 1277d4debf4e9ab6ba8f8de60b6ff785a52f007b..6cd4479611b695b9a13fae9d6a3d113cf09e4ade 100644
--- a/Source/core/animation/AnimationTimeline.h
+++ b/Source/core/animation/AnimationTimeline.h
@@ -138,6 +138,7 @@ private:
         AnimationTimelineTiming(AnimationTimeline* timeline)
             : m_timeline(timeline)
             , m_timer(this, &AnimationTimelineTiming::timerFired)
+            , m_currentScheduledTime(std::numeric_limits<double>::infinity())
         {
             ASSERT(m_timeline);
         }
@@ -153,6 +154,7 @@ private:
     private:
         RawPtrWillBeMember<AnimationTimeline> m_timeline;
         Timer<AnimationTimelineTiming> m_timer;
+        double m_currentScheduledTime;
     };
 
     friend class AnimationAnimationTimelineTest;