QWebEngineUrlRequestInfo::firstPartyUrl() returns empty url

When intercepting UrlRequest the first party url is empty if the request url and the first party url doesn’t have the same origin.

It worked before the 69-based update. The issue is triggered by: https://chromium.googlesource.com/chromium/src/+/07fbae50670ea44e35e1d554db1bbece7fe3711f%5E%21/content/browser/frame_host/navigation_request.cc

The qrc:/ urls work since https://codereview.qt-project.org/#/c/240565/

The attached example demonstrate the problem (main.cpp): the second url request is triggered by an iframe and the first party url of this request is not available because the local pages do not have the same origin.

Proposed solution: replace URLRequest::site_for_cookies() usage in network_delegate_qt.cpp because site_for_cookies() is not supposed to work for non-same-origin requests.