Implement DocumentTimeline constructor

third_party/WebKit/Source/core/animation/DocumentTimeline.idl

 // Copyright 2016 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.
 
 // https://w3c.github.io/web-animations/#the-documenttimeline-interface
 
 [
+    Constructor(optional DocumentTimelineOptions options),
+    ConstructorCallWith=ExecutionContext,
     RuntimeEnabled=WebAnimationsAPI
 ] interface DocumentTimeline : AnimationTimeline {
 };