Implement MouseEvent buttons attribute.

Source/platform/PlatformMouseEvent.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 43 matching lines @@
 
     PlatformMouseEvent(const IntPoint& position, const IntPoint& globalPosition, MouseButton button, PlatformEvent::Type type, int clickCount, Modifiers modifiers, double timestamp)
         : PlatformEvent(type, modifiers, timestamp)
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
         , m_clickCount(clickCount)
         , m_synthesized(RealOrIndistinguishable)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     PlatformMouseEvent(const IntPoint& position, const IntPoint& globalPosition, MouseButton button, PlatformEvent::Type type, int clickCount, Modifiers modifiers, SyntheticEventType synthesized, double timestamp)
         : PlatformEvent(type, modifiers, timestamp)
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
         , m_clickCount(clickCount)
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     PlatformMouseEvent(const IntPoint& position, const IntPoint& globalPosition, MouseButton button, PlatformEvent::Type type, int clickCount, bool shiftKey, bool ctrlKey, bool altKey, bool metaKey, SyntheticEventType synthesized, double timestamp)
         : PlatformEvent(type, shiftKey, ctrlKey, altKey, metaKey, timestamp)
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
         , m_clickCount(clickCount)
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     const IntPoint& position() const { return m_position; }
     const IntPoint& globalPosition() const { return m_globalPosition; }
     const IntPoint& movementDelta() const { return m_movementDelta; }
 
     MouseButton button() const { return m_button; }
     int clickCount() const { return m_clickCount; }
     unsigned modifierFlags() const { return m_modifierFlags; }
     bool fromTouch() const { return m_synthesized == FromTouch; }
     SyntheticEventType syntheticEventType() const { return m_synthesized; }
 
+private:
+    void addModifierFromButton(MouseButton button)
+    {
+        if (!fromTouch() || button == NoButton)
+            return;
+
+        const unsigned map[] = {
+            LeftButtonDown, MiddleButtonDown, RightButtonDown
+        };
+        m_modifiers |= map[button];
+    }
+
 protected:
     IntPoint m_position;
     IntPoint m_globalPosition;
     IntPoint m_movementDelta;
     MouseButton m_button;
     int m_clickCount;
     SyntheticEventType m_synthesized;
     unsigned m_modifierFlags;
 };
 
 } // namespace blink
 
 #endif // PlatformMouseEvent_h