Assert failure on qwebenginepage.cpp when context menu event handled on event filter

Q_ASSERT on qwebenginepage.cpp fails when the QEvent::ContextMenu event is handled on the event filter.

Example code from the event filter that consumes QEvent::ContextMenu (full example code attached):

    bool eventFilter(QObject* watched, QEvent* event)
    {
        QWebEngineView* view = qobject_cast<QWebEngineView*>(watched);

        if (view) {
            if (event->type() == QEvent::ContextMenu) {
                qDebug()<< "Do something for QEvent::ContextMenu.";
                return true;
            }

Attached example application demonstrates the use case (event filter is found from mainwindow.h):

• run the example on debug mode and press the right mouse key --> ASSERT failure:

  Starting D:\mycases\build-bser-Desktop_Qt_5_5_1_MSVC2013_64bit-Debug\debug\bser.exe...
  Do something for QEvent::ContextMenu.
  ASSERT: "view->d_func()->m_pendingContextMenuEvent" in file api\qwebenginepage.cpp, line 728