Add IntersectionObserverEntry.intersectionRatio attribute.

third_party/WebKit/Source/core/dom/IntersectionObserverEntry.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://github.com/slightlyoff/IntersectionObserver
 
 [
     GarbageCollected,
     RuntimeEnabled=IntersectionObserver,
 ] interface IntersectionObserverEntry {
     readonly attribute ClientRect boundingClientRect;
     readonly attribute ClientRect intersectionRect;
+    readonly attribute double intersectionRatio;
     readonly attribute ClientRect? rootBounds;
     readonly attribute Element target;
     readonly attribute DOMHighResTimeStamp time;
 };