Sync IntersectionObserver IDL with the spec

third_party/WebKit/Source/core/dom/IntersectionObserverInit.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): link to final spec
+// https://wicg.github.io/IntersectionObserver/#intersection-observer-init
 
 [
     RuntimeEnabled=IntersectionObserver
 ] dictionary IntersectionObserverInit {
-    Element? root;
-    DOMString rootMargin;
-    (double or double[]) threshold = 0;
+    Element? root = null;
+    DOMString rootMargin = "0px";
+    (double or sequence<double>) threshold = 0;
 };