Support buttons attribute for drag event (blink side).

Source/core/page/DragController.cpp

 /*
  * Copyright (C) 2007, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 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 93 matching lines @@
     case DragSourceActionNone:
         return false;
     }
     // Make sure MSVC doesn't complain that not all control paths return a value.
     return false;
 }
 #endif
 
 static PlatformMouseEvent createMouseEvent(DragData* dragData)
 {
-    int keyState = dragData->modifierKeyState();
-    bool shiftKey = static_cast<bool>(keyState & PlatformEvent::ShiftKey);
-    bool ctrlKey = static_cast<bool>(keyState & PlatformEvent::CtrlKey);
-    bool altKey = static_cast<bool>(keyState & PlatformEvent::AltKey);
-    bool metaKey = static_cast<bool>(keyState & PlatformEvent::MetaKey);
-
     return PlatformMouseEvent(dragData->clientPosition(), dragData->globalPosition(),
-        LeftButton, PlatformEvent::MouseMoved, 0, shiftKey, ctrlKey, altKey,
-        metaKey, PlatformMouseEvent::RealOrIndistinguishable, currentTime());
+        LeftButton, PlatformEvent::MouseMoved, 0,
+        static_cast<PlatformEvent::Modifiers>(dragData->modifierKeyState()),
+        PlatformMouseEvent::RealOrIndistinguishable, currentTime());
 }
 
 static PassRefPtrWillBeRawPtr<DataTransfer> createDraggingDataTransfer(DataTransferAccessPolicy policy, DragData* dragData)
 {
     return DataTransfer::create(DataTransfer::DragAndDrop, policy, dragData->platformData());
 }
 
 DragController::DragController(Page* page, DragClient* client)
     : m_page(page)
     , m_client(client)
@@ skipping 853 matching lines @@
 
 void DragController::trace(Visitor* visitor)
 {
     visitor->trace(m_page);
     visitor->trace(m_documentUnderMouse);
     visitor->trace(m_dragInitiator);
     visitor->trace(m_fileInputElementUnderMouse);
 }
 
 } // namespace blink