Implement MouseEvent buttons attribute.

Source/platform/PlatformEvent.h

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 59 matching lines @@
         TouchMove,
         TouchEnd,
         TouchCancel,
     };
 
     enum Modifiers {
         AltKey      = 1 << 0,
         CtrlKey     = 1 << 1,
         MetaKey     = 1 << 2,
         ShiftKey    = 1 << 3,
+
+        LeftButtonDown   = 1 << 6,
+        MiddleButtonDown = 1 << 7,
+        RightButtonDown  = 1 << 8,
     };
 
     Type type() const { return static_cast<Type>(m_type); }
 
     bool shiftKey() const { return m_modifiers & ShiftKey; }
     bool ctrlKey() const { return m_modifiers & CtrlKey; }
     bool altKey() const { return m_modifiers & AltKey; }
     bool metaKey() const { return m_modifiers & MetaKey; }
 
     unsigned modifiers() const { return m_modifiers; }
@@ skipping 44 matching lines @@
     }
 
     unsigned m_type;
     unsigned m_modifiers;
     double m_timestamp;
 };
 
 } // namespace blink
 
 #endif // PlatformEvent_h