Support buttons attribute for drag event (blink side).

Source/core/page/DragData.h

 /*
  * Copyright (C) 2007 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 50 matching lines @@
     DragOperation draggingSourceOperationMask() const { return m_draggingSourceOperationMask; }
     bool containsURL(FilenameConversionPolicy filenamePolicy = ConvertFilenames) const;
     bool containsPlainText() const;
     bool containsCompatibleContent() const;
     String asURL(FilenameConversionPolicy filenamePolicy = ConvertFilenames, String* title = nullptr) const;
     String asPlainText() const;
     void asFilePaths(Vector<String>&) const;
     PassRefPtrWillBeRawPtr<DocumentFragment> asFragment(LocalFrame*, PassRefPtrWillBeRawPtr<Range> context, bool allowPlainText, bool& chosePlainText) const;
     bool canSmartReplace() const;
     bool containsFiles() const;
-    int modifierKeyState() const;
+    int modifiers() const;
 
     String droppedFileSystemId() const;
 
 private:
     IntPoint m_clientPosition;
     IntPoint m_globalPosition;
     DataObject* m_platformDragData;
     DragOperation m_draggingSourceOperationMask;
     DragApplicationFlags m_applicationFlags;
 };
 
 }
 
 #endif // !DragData_h