Support buttons attribute for drag event (blink side).

Source/core/clipboard/DataObject.h

 /*
  * Copyright (c) 2008, 2009, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 76 matching lines @@
     Vector<String> filenames() const;
     void addFilename(const String& filename, const String& displayName);
 
     // Used for dragging in filesystem from the desktop.
     void setFilesystemId(const String& fileSystemId) { m_filesystemId = fileSystemId; }
     const String& filesystemId() const { ASSERT(!m_filesystemId.isEmpty()); return m_filesystemId; }
 
     // Used to handle files (images) being dragged out.
     void addSharedBuffer(const String& name, PassRefPtr<SharedBuffer>);
 
-    int modifierKeyState() const { return m_modifierKeyState; }
-    void setModifierKeyState(int modifierKeyState) { m_modifierKeyState = modifierKeyState; }
+    int modifiers() const { return m_modifiers; }
+    void setModifiers(int modifiers) { m_modifiers = modifiers; }
 
     DECLARE_TRACE();
 
     WebDragData toWebDragData();
 
 private:
     DataObject();
 
     PassRefPtrWillBeRawPtr<DataObjectItem> findStringItem(const String& type) const;
     bool internalAddStringItem(PassRefPtrWillBeRawPtr<DataObjectItem>);
     void internalAddFileItem(PassRefPtrWillBeRawPtr<DataObjectItem>);
 
     WillBeHeapVector<RefPtrWillBeMember<DataObjectItem> > m_itemList;
 
-    // State of Shift/Ctrl/Alt/Meta keys.
-    int m_modifierKeyState;
+    // State of Shift/Ctrl/Alt/Meta keys and Left/Right/Middle mouse buttons
+    int m_modifiers;
     String m_filesystemId;
 };
 
 } // namespace blink
 
 #endif