Support buttons attribute for drag event (blink side).

Source/core/clipboard/DataObject.cpp

 /*
  * Copyright (c) 2008, 2009, 2012 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 202 matching lines @@
 {
     internalAddFileItem(DataObjectItem::createFromFile(File::createForUserProvidedFile(filename, displayName)));
 }
 
 void DataObject::addSharedBuffer(const String& name, PassRefPtr<SharedBuffer> buffer)
 {
     internalAddFileItem(DataObjectItem::createFromSharedBuffer(name, buffer));
 }
 
 DataObject::DataObject()
-    : m_modifierKeyState(0)
+    : m_modifiers(0)
 {
 }
 
 DataObject::DataObject(const DataObject& other)
     : m_itemList(other.m_itemList)
-    , m_modifierKeyState(0)
+    , m_modifiers(0)
 {
 }
 
 PassRefPtrWillBeRawPtr<DataObjectItem> DataObject::findStringItem(const String& type) const
 {
     for (size_t i = 0; i < m_itemList.size(); ++i) {
         if (m_itemList[i]->kind() == DataObjectItem::StringKind && m_itemList[i]->type() == type)
             return m_itemList[i];
     }
     return nullptr;
@@ skipping 19 matching lines @@
 
 void DataObject::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_itemList);
     HeapSupplementable<DataObject>::trace(visitor);
 #endif
 }
 
 } // namespace blink