Implement MouseEvent buttons attribute.

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * 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.
@@ skipping 1617 matching lines @@
 
 
 bool EventHandler::dispatchDragEvent(const AtomicString& eventType, Node* dragTarget, const PlatformMouseEvent& event, DataTransfer* dataTransfer)
 {
     FrameView* view = m_frame->view();
 
     // FIXME: We might want to dispatch a dragleave even if the view is gone.
     if (!view)
         return false;
 
+    // FIXME: The drag event have to support for |buttons| attribute because
+    // the event is derived from mouse event. Please see crbug.com/276941.
     RefPtrWillBeRawPtr<MouseEvent> me = MouseEvent::create(eventType,
         true, true, m_frame->document()->domWindow(),
         0, event.globalPosition().x(), event.globalPosition().y(), event.position().x(), event.position().y(),
         event.movementDelta().x(), event.movementDelta().y(),
         event.ctrlKey(), event.altKey(), event.shiftKey(), event.metaKey(),
-        0, nullptr, dataTransfer);
+        0, 0, nullptr, dataTransfer);
 
     dragTarget->dispatchEvent(me.get(), IGNORE_EXCEPTION);
     return me->defaultPrevented();
 }
 
 static bool targetIsFrame(Node* target, LocalFrame*& frame)
 {
     if (!isHTMLFrameElementBase(target))
         return false;
 
@@ skipping 2206 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink