Animations: Add the effect clipping primitive to web animations

Source/core/animation/Animation.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 29 matching lines @@
     // TODO(dstockwell): Rename source to effect.
     [RuntimeEnabled=WebAnimationsAPI] attribute AnimationEffectReadOnly? source;
     [Measure] attribute double?    startTime;
     [Measure] attribute double?    currentTime;
     [Measure] attribute double     playbackRate;
     [Measure] readonly attribute AnimationPlayState playState;
     [Measure, RaisesException] void finish();
     [Measure] void play();
     [Measure] void pause();
     [Measure] void reverse();
+    [RuntimeEnabled=WebAnimationsAPI] attribute double startClip;
+    [RuntimeEnabled=WebAnimationsAPI] attribute double endClip;
 
     [Measure] void cancel();
     [Measure] attribute EventHandler onfinish;
     [RuntimeEnabled=WebAnimationsAPI, CallWith=ScriptState] readonly attribute Promise<Animation> finished;
     [RuntimeEnabled=WebAnimationsAPI, CallWith=ScriptState] readonly attribute Promise<Animation> ready;
 };