Clear the WA_QuitOnClose flag from EvalMessageBox.

The evaluation message box caused  the QtWebProcess
to quit due to the fact that the process has no other
windows and EvalMessageBox was both set to be closed
on quit and ended up being the last window in the process.

Change-Id: Iad6461d014258fdc5fade7dafe48da33903377bb
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>

src/corelib/kernel/qtcore_eval.cpp

@@ -496,6 +496,7 @@ public:
             setParent(parentWidget(), Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint);
             QTimer::singleShot(7000, this, SLOT(close()));
             setAttribute(Qt::WA_DeleteOnClose);
+            setAttribute(Qt::WA_QuitOnClose, false);
         }
 
         setFixedSize(sizeHint());