Support buttons attribute for drag event (blink side).

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 4b01ae2602c2dced9c50c9ea9f0c44d23dd2bccc..d374bcf8d2a8d6108fe026e6de5993a67006941a 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -1659,14 +1659,12 @@ 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, 0, nullptr, dataTransfer);
+        0, MouseEvent::platformModifiersToButtons(event.modifiers()), nullptr, dataTransfer);
 
     dragTarget->dispatchEvent(me.get(), IGNORE_EXCEPTION);
     return me->defaultPrevented();