Declare onFinish event handler for 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 12 matching lines @@
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     RuntimeEnabled=WebAnimationsAPI,
-] interface AnimationPlayer {
+] interface AnimationPlayer : EventTarget {
              attribute TimedItem? source;
              attribute double     startTime;
              attribute double     currentTime;
     readonly attribute double     timeLag;
              attribute double     playbackRate;
     readonly attribute boolean    paused;
     readonly attribute boolean    finished;
     void cancel();
     [RaisesException] void finish();
     void play();
     void pause();
     void reverse();
+
+    attribute EventHandler onfinish;
 };