Implement attribute getters for IntersectionObserver.

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): add link to final spec.
 callback IntersectionObserverCallback = void (sequence<IntersectionObserverEntry> entries, IntersectionObserver observer);
 
 // TODO(szager): Update the doc link when the spec has an official home.
 // https://github.com/slightlyoff/IntersectionObserver
 [
     CustomConstructor(IntersectionObserverCallback callback, IntersectionObserverInit options),
     GarbageCollected,
     RuntimeEnabled=IntersectionObserver,
 ] interface IntersectionObserver {
     [RaisesException] void observe(Element target);
     [RaisesException] void unobserve(Element target);
     void disconnect();
     sequence<IntersectionObserverEntry> takeRecords();
+    readonly attribute Element? root;
+    readonly attribute DOMString rootMargin;
+    readonly attribute double[] thresholds;
 };