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

Source/core/events/MouseEventInit.idl

 // Copyright 2014 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/uievents/#idl-def-MouseEventInit
 
 dictionary MouseEventInit : EventModifierInit {
     long screenX = 0;
     long screenY = 0;
     long clientX = 0;
     long clientY = 0;
-    // TODO(philipj): button should be short, not unsigned short.
-    unsigned short button = 0;
+    short button = 0;
     unsigned short buttons = 0;
     EventTarget? relatedTarget = null;
 
     // Pointer Lock
     // https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-mouseeventinit-dictionary
     long movementX = 0;
     long movementY = 0;
 };