Window.showModalDialog() is not supported since Chromium 37, so we can
remove the showModalDialog test case.

If the view is not visible, the width and the height of the requested
geometry is always 0 in tst_QWebEnginePage::openWindowDefaultSize.

Change-Id: I420828815bc9528070ca58a9e42dea44517aeafc
Reviewed-by: Andras Becsi <andras.becsi@digia.com>