Gated behind the feature flag `dom_navigator_sendbeacon_enabled` as the
`keep-alive` fetch parameter is crucial for real-life use cases such as
analytics requests.
Part of #4577
Part of #38302
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>
Propagate through documents a flag that represents if any of the
ancestor navigables has a potentially trustworthy origin.
The "potentially trustworthy origin" concept appears to have gotten
confused in a couple of places and we were instead testing if a URL had
"potentially trustworthy" properties.
The main test for the ancestor navigables is
[mixed-content/nested-iframes](https://github.com/web-platform-tests/wpt/blob/master/mixed-content/nested-iframes.window.js)
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---------
Signed-off-by: Sebastian C <sebsebmc@gmail.com>
