Document the tracking bugs of some IDL FIXMEs

Source/core/events/MouseEvent.idl

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 53 matching lines @@
     [MeasureAs=MouseEventOffsetX] readonly attribute long offsetX;
     [MeasureAs=MouseEventOffsetY] readonly attribute long offsetY;
     [MeasureAs=MouseEventX] readonly attribute long x;
     [MeasureAs=MouseEventY] readonly attribute long y;
     [MeasureAs=MouseEventFromElement] readonly attribute Node fromElement;
     [MeasureAs=MouseEventToElement] readonly attribute Node toElement;
     [MeasureAs=MouseEventWhich] readonly attribute long which;
     [DeprecateAs=PrefixedMouseEventMovementX, ImplementedAs=movementX] readonly attribute long webkitMovementX;
     [DeprecateAs=PrefixedMouseEventMovementY, ImplementedAs=movementY] readonly attribute long webkitMovementY;
 
-    // FIXME: this belongs on the DragEvent interface
+    // FIXME: dataTransfer belongs on the DragEvent interface. crbug.com/498504
     readonly attribute DataTransfer dataTransfer;
 };