Declare onFinish event handler for AnimationPlayer

Source/core/events/AnimationPlayerEvent.idl

Index: Source/core/events/AnimationPlayerEvent.idl
diff --git a/Source/core/events/AnimationPlayerEvent.idl b/Source/core/events/AnimationPlayerEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..a719ec75f0cca6276dbfaf2029dbdf2f9dad16b2
--- /dev/null
+++ b/Source/core/events/AnimationPlayerEvent.idl
@@ -0,0 +1,12 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[
+    EventConstructor,
+    RuntimeEnabled=WebAnimationsAPI,
+] interface AnimationPlayerEvent : Event {
+    [InitializedByEventConstructor] readonly attribute double currentTime;
+    [InitializedByEventConstructor] readonly attribute double timelineTime;
+};
+