Fix button event on mouse events.

third_party/WebKit/Source/core/events/MouseEvent.h

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008 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.
@@ skipping 73 matching lines @@
                       bool ctrl_key,
                       bool alt_key,
                       bool shift_key,
                       bool meta_key,
                       short button,
                       EventTarget* related_target,
                       unsigned short buttons = 0);
 
   // WinIE uses 1,4,2 for left/middle/right but not for click (just for
   // mousedown/up, maybe others), but we will match the standard DOM.
-  virtual short button() const { return button_ == -1 ? 0 : button_; }
+  virtual short button() const;
   unsigned short buttons() const { return buttons_; }
   bool ButtonDown() const { return button_ != -1; }
   EventTarget* relatedTarget() const { return related_target_.Get(); }
   void SetRelatedTarget(EventTarget* related_target) {
     related_target_ = related_target;
   }
   SyntheticEventType GetSyntheticEventType() const {
     return synthetic_event_type_;
   }
   const String& region() const { return region_; }
@@ skipping 166 matching lines @@
   MouseEvent& Event() const;
 
   DispatchEventResult DispatchEvent(EventDispatcher&) const override;
 };
 
 DEFINE_EVENT_TYPE_CASTS(MouseEvent);
 
 }  // namespace blink
 
 #endif  // MouseEvent_h