IntersectionObserver: add thresholds attribute.

third_party/WebKit/Source/core/dom/IntersectionObserver.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.
 
 // TODO(szager): Update the doc link when the spec has an official home; for now it's:
 //   https://github.com/WICG/IntersectionObserver
 
 callback IntersectionObserverCallback = void (sequence<IntersectionObserverEntry> entries, IntersectionObserver observer);
 
-// TODO(szager): Add the thresholds attribute when FrozenArray has been implemented for IDL:
-//   readonly attribute FrozenArray<double> thresholds;
 [
     CustomConstructor(IntersectionObserverCallback callback, IntersectionObserverInit options),
     Custom=VisitDOMWrapper,
     MeasureAs=IntersectionObserver_Constructor,
     RuntimeEnabled=IntersectionObserver,
 ] interface IntersectionObserver {
     void observe(Element target);
     void unobserve(Element target);
     void disconnect();
     sequence<IntersectionObserverEntry> takeRecords();
     readonly attribute Element? root;
     readonly attribute DOMString rootMargin;
+    readonly attribute FrozenArray<double> thresholds;
 };