Web Animations: Expose timeline currentTime in IDL

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 200568385440ed73b6229bb3e8dce711e0272ba2..f2201a0661ec8d361fd6139e215e577e6cb21e30 100644
--- a/Source/core/animation/DocumentTimeline.h
+++ b/Source/core/animation/DocumentTimeline.h
@@ -80,6 +80,7 @@ public:
     bool hasStarted() const { return !isNull(m_zeroTime); }
     bool hasPendingUpdates() const { return !m_playersNeedingUpdate.isEmpty(); }
     double zeroTime() const { return m_zeroTime; }
+    double currentTime(bool& isNull);
     double currentTime();
     double effectiveTime();
     void pauseAnimationsForTesting(double);