Web Animations: Expose timeline currentTime in IDL

Source/core/animation/DocumentTimeline.cpp

Index: Source/core/animation/DocumentTimeline.cpp
diff --git a/Source/core/animation/DocumentTimeline.cpp b/Source/core/animation/DocumentTimeline.cpp
index 77ef592d566ff005da38639a7d62257444a653db..88bc50d0c12a24df1522f541f1680182e1440b3a 100644
--- a/Source/core/animation/DocumentTimeline.cpp
+++ b/Source/core/animation/DocumentTimeline.cpp
@@ -144,11 +144,21 @@ void DocumentTimeline::DocumentTimelineTiming::serviceOnNextFrame()
         m_timeline->m_document->view()->scheduleAnimation();
 }
 
-double DocumentTimeline::currentTime()
+double DocumentTimeline::currentTime(bool& isNull)
 {
-    if (!m_document)
+    if (!m_document) {
+        isNull = true;
         return std::numeric_limits<double>::quiet_NaN();
-    return m_document->animationClock().currentTime() - m_zeroTime;
+    }
+    double result = m_document->animationClock().currentTime() - m_zeroTime;
+    isNull = std::isnan(result);
+    return result;
+}
+
+double DocumentTimeline::currentTime()
+{
+    bool isNull;
+    return currentTime(isNull);
 }
 
 double DocumentTimeline::effectiveTime()