Added spec link to PointerEvent

third_party/WebKit/Source/core/events/PointerEventInit.idl

 // Copyright 2015 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://w3c.github.io/pointerevents/#pointerevent-interface
 
 dictionary PointerEventInit : MouseEventInit {
     long      pointerId = 0;
     double    width = 1;
     double    height = 1;
     float     pressure = 0;
     long      tiltX = 0;
     long      tiltY = 0;
     float     tangentialPressure = 0;
     long      twist = 0;
     DOMString pointerType = "";
     boolean   isPrimary = false;
 
+    // https://w3c.github.io/pointerevents/extension.html#extensions-to-the-pointerevent-interface
     [RuntimeEnabled=CoalescedEvents] sequence<PointerEvent> coalescedEvents = [];
 };