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

Source/core/animation/AnimationPlayer.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 18 matching lines @@
  */
 
 enum AnimationPlayState { "idle", "pending", "running", "paused", "finished" };
 
 [
     NoInterfaceObject,
     WillBeGarbageCollected,
     ActiveDOMObject,
 ] interface AnimationPlayer : EventTarget {
     [RuntimeEnabled=WebAnimationsAPI]                      attribute AnimationNode? source;
-    [RuntimeEnabled=WebAnimationsPlaybackControl]          attribute double     startTime;
-    [RuntimeEnabled=WebAnimationsPlaybackControl]          attribute double     currentTime;
+    [RuntimeEnabled=WebAnimationsPlaybackControl]          attribute double?    startTime;
+    [RuntimeEnabled=WebAnimationsPlaybackControl]          attribute double?    currentTime;
     [RuntimeEnabled=WebAnimationsPlaybackControl]          attribute double     playbackRate;
     [RuntimeEnabled=WebAnimationsPlaybackControl] readonly attribute AnimationPlayState playState;
     [RuntimeEnabled=WebAnimationsPlaybackControl, RaisesException] void finish();
     [RuntimeEnabled=WebAnimationsPlaybackControl] void play();
     [RuntimeEnabled=WebAnimationsPlaybackControl] void pause();
     [RuntimeEnabled=WebAnimationsPlaybackControl] void reverse();
 
     void cancel();
     [MeasureAs=AnimationPlayerFinishEvent] attribute EventHandler onfinish;
 };