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

[szager1, 2016/10/01 00:03:30]

Can you remove RuntimeEnabled=IntersectionObserver from the rest of the files in
this CL?

[foolip, 2016/10/01 15:23:20]

I actually removed it since [RuntimeEnabled] doesn't do anything for
dicionaries, but instead I'll restore this and remove it from all dictionaries
separately.

[foolip, 2016/10/01 15:33:18]

https://codereview.chromium.org/2390483002

I'll leave dropping the IntersectionObserver REF entirely to someone else :)

-] dictionary IntersectionObserverInit {
-    Element? root;
-    DOMString rootMargin;
-    (double or double[]) threshold = 0;
+dictionary IntersectionObserverInit {
+    Element? root = null;
+    DOMString rootMargin = "0px";
+    (double or sequence<double>) threshold = 0;
 };