Implement MouseEvent buttons attribute.

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index a60edf238da7a23f981fc62823e51ea314077dbd..b864ae113ede18767b7da258c8111ec633a7e5ca 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -1635,12 +1635,14 @@ bool EventHandler::dispatchDragEvent(const AtomicString& eventType, Node* dragTa
     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();