Added spec link to PointerEvent

third_party/WebKit/Source/core/events/PointerEvent.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
 
 [
     Constructor(DOMString type, optional PointerEventInit eventInitDict),
     RuntimeEnabled=PointerEvent,
 ] interface PointerEvent : MouseEvent {
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute long      pointerId;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute double    width;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute double    height;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute float     pressure;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute long      tiltX;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute long      tiltY;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute float     tangentialPressure;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute long      twist;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute DOMString pointerType;
     [MeasureAs=PointerEventAttributeCount]   readonly    attribute boolean   isPrimary;
 
+    // https://w3c.github.io/pointerevents/extension.html#extensions-to-the-pointerevent-interface
     [RuntimeEnabled=CoalescedEvents] sequence<PointerEvent> getCoalescedEvents();
 };