Change MouseEvent.button to be 'short' instead of 'unsigned short'

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 13 matching lines @@
     ConstructorCallWith=ScriptState,
 ] interface MouseEvent : UIEvent {
     readonly attribute long             screenX;
     readonly attribute long             screenY;
     readonly attribute long             clientX;
     readonly attribute long             clientY;
     readonly attribute boolean          ctrlKey;
     readonly attribute boolean          shiftKey;
     readonly attribute boolean          altKey;
     readonly attribute boolean          metaKey;
-    // TODO(philipj): button should be short, not unsigned short.
-    readonly attribute unsigned short   button;
+    readonly attribute short            button;
     readonly attribute unsigned short   buttons;
     readonly attribute EventTarget?     relatedTarget;
     // TODO(philipj): boolean getModifierState(DOMString keyArg);
 
     // https://w3c.github.io/uievents/#idl-interface-MouseEvent-initializers
     // TODO(philipj): None of the initMouseEvent() arguments should be optional.
     [CallWith=ScriptState] void initMouseEvent([Default=Undefined] optional DOMString type,
                                        [Default=Undefined] optional boolean bubbles,
                                        [Default=Undefined] optional boolean cancelable,
                                        [Default=Undefined] optional Window? view,
@@ skipping 21 matching lines @@
     [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: dataTransfer belongs on the DragEvent interface. crbug.com/498504
     readonly attribute DataTransfer dataTransfer;
 };