In order to ensure WebDriver continues working in a post site origin
isolation world, we should write some cross-origin <iframe> tests
for WPT.

I believe the existing tests all serve content from web.platform
or an equivalent loopback domain. wptserve has support for multiple
origins.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1555701 for
running the Wd job with Fission enabled. The two are strictly not
dependencies.

Bugbug thinks this bug is a task, but please change it back in case of error.

Adds two simple cross-origin WPT tests for WebDriver, testing
that WebDriver's Switch To Frame command can transcend frames from
different site origins.

The justification for adding these tests to WPT is two-fold: firstly,
browsers enforce strict cross-origin checks on frames and we should
test for that scenario; but secondly, and perhaps more importantly,
modern browsers are moving towards process isolation on a site-origin
basis and this is expected to cause problems for WebDriver.

Testing this behaviour will ensure any attempts at switching on
process isolation on site-origin basis (i.e. "Fission") will test
WebDriver's behaviour.

https://hg.mozilla.org/mozilla-central/rev/8f179514723d