Implement MouseEvent buttons attribute.

Source/platform/PlatformWheelEvent.h

 /*
  * Copyright (C) 2004, 2005, 2006, 2009 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 86 matching lines @@
     float deltaX() const { return m_deltaX; }
     float deltaY() const { return m_deltaY; }
 
     float wheelTicksX() const { return m_wheelTicksX; }
     float wheelTicksY() const { return m_wheelTicksY; }
 
     PlatformWheelEventGranularity granularity() const { return m_granularity; }
 
     bool hasPreciseScrollingDeltas() const { return m_hasPreciseScrollingDeltas; }
     void setHasPreciseScrollingDeltas(bool b) { m_hasPreciseScrollingDeltas = b; }
+
+    unsigned short buttons() const
+    {
+        unsigned short value = 0;
+
+        if (m_modifiers & LeftButtonDown)
+            value |= 1;
+        if (m_modifiers & RightButtonDown)
+            value |= 2;
+        if (m_modifiers & MiddleButtonDown)
+            value |= 4;
+
+        return value;
+    }
 #if OS(MACOSX)
     PlatformWheelEventPhase phase() const { return m_phase; }
     PlatformWheelEventPhase momentumPhase() const { return m_momentumPhase; }
     bool useLatchedEventNode() const { return m_momentumPhase == PlatformWheelEventPhaseBegan || m_momentumPhase == PlatformWheelEventPhaseChanged; }
     bool canRubberbandLeft() const { return m_canRubberbandLeft; }
     bool canRubberbandRight() const { return m_canRubberbandRight; }
 #else
     bool useLatchedEventNode() const { return false; }
 #endif
 
@@ skipping 10 matching lines @@
     PlatformWheelEventPhase m_phase;
     PlatformWheelEventPhase m_momentumPhase;
     bool m_canRubberbandLeft;
     bool m_canRubberbandRight;
 #endif
 };
 
 } // namespace blink
 
 #endif // PlatformWheelEvent_h