Implement MouseEvent buttons attribute.

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 11 matching lines @@
 ] interface MouseEvent : UIEvent {
     [InitializedByEventConstructor] readonly attribute long             screenX;
     [InitializedByEventConstructor] readonly attribute long             screenY;
     [InitializedByEventConstructor] readonly attribute long             clientX;
     [InitializedByEventConstructor] readonly attribute long             clientY;
     [InitializedByEventConstructor] readonly attribute boolean          ctrlKey;
     [InitializedByEventConstructor] readonly attribute boolean          shiftKey;
     [InitializedByEventConstructor] readonly attribute boolean          altKey;
     [InitializedByEventConstructor] readonly attribute boolean          metaKey;
     [InitializedByEventConstructor] readonly attribute unsigned short   button;
+    [RuntimeEnabled=MouseEventButtonsAttribute, InitializedByEventConstructor] readonly attribute unsigned short   buttons;
     [InitializedByEventConstructor] readonly attribute EventTarget?     relatedTarget;
     [MeasureAs=MouseEventMovementX] readonly attribute long             movementX;
     [MeasureAs=MouseEventMovementY] readonly attribute long             movementY;
     [MeasureAs=PrefixedMouseEventMovementX, ImplementedAs=movementX] readonly attribute long webkitMovementX;
     [MeasureAs=PrefixedMouseEventMovementY, ImplementedAs=movementY] readonly attribute long webkitMovementY;
 
      void initMouseEvent([Default=Undefined] optional DOMString type,
                                        [Default=Undefined] optional boolean canBubble,
                                        [Default=Undefined] optional boolean cancelable,
                                        [Default=Undefined] optional Window view,
@@ skipping 14 matching lines @@
     [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;
 
     // FIXME: this belongs on the DragEvent interface
     readonly attribute DataTransfer dataTransfer;
 };