Web Animations: Use null for unresolved startTime and currentTime in AnimationPlayer

Source/core/animation/AnimationPlayer.h

Index: Source/core/animation/AnimationPlayer.h
diff --git a/Source/core/animation/AnimationPlayer.h b/Source/core/animation/AnimationPlayer.h
index 8cdfc587e056eb663721cb245f08012bb29f1e1b..e987097d1c611a93a6dbb23a94cb622ab74946d6 100644
--- a/Source/core/animation/AnimationPlayer.h
+++ b/Source/core/animation/AnimationPlayer.h
@@ -70,6 +70,7 @@ public:
 
     void cancel();
 
+    double currentTime(bool& isNull);
     double currentTime();
     void setCurrentTime(double newCurrentTime);
 
@@ -110,6 +111,7 @@ public:
 
     double calculateStartTime(double currentTime) const;
     bool hasStartTime() const { return !isNull(m_startTime); }
+    double startTime(bool& isNull) const;
     double startTime() const;
     double startTimeInternal() const { return m_startTime; }
     void setStartTime(double);