Support buttons attribute for drag event (blink side).

Source/core/page/DragData.cpp

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2013 Google Inc.
  *
  * 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
@@ skipping 57 matching lines @@
     else if (filenamePolicy == ConvertFilenames && containsFiles())
         url = filePathToURL(m_platformDragData->filenames()[0]);
     return url;
 }
 
 bool DragData::containsFiles() const
 {
     return m_platformDragData->containsFilenames();
 }
 
-int DragData::modifierKeyState() const
+int DragData::modifiers() const
 {
-    return m_platformDragData->modifierKeyState();
+    return m_platformDragData->modifiers();
 }
 
 void DragData::asFilePaths(Vector<String>& result) const
 {
     const Vector<String>& filenames = m_platformDragData->filenames();
     for (size_t i = 0; i < filenames.size(); ++i) {
         if (!filenames[i].isEmpty())
             result.append(filenames[i]);
     }
 }
@@ skipping 54 matching lines @@
 
     return nullptr;
 }
 
 String DragData::droppedFileSystemId() const
 {
     return m_platformDragData->filesystemId();
 }
 
 } // namespace blink