Custom user agent is not carried over when opening a new tab/page via HTML form button

Very similar to QTBUG-48891 (which has been fixed): when opening a new page (ctrl+click or middle click) from HTML form button, the custom user agent of the source page is not kept.

To reproduce this issue:

• compile and run QtWebEngine demobrowser example,
• setup a custom UA (Edit > Preferences > Advanced),
• download and load the attached index.html in demobrowser,
• ctrl+click on the link, the reported UA is correct,
• ctrl+click on the button, the reported UA is wrong,
• click (no new page) on the button, the UA is correct.