Vague behaviour with execute async script and navigations

[gsnedders]

Partly motivated by web-platform-tests/wpt#48825:

What should be the behaviour of:

def test_window_navigates(session):
    session.url = "about:blank"
    response = execute_async_script(session, """
        let resolve = arguments[0];
        setTimeout(() => resolve(), 10000);
        location = "about:blank?new_url";
        """)
    assert_error(response, "script timeout")

Currently:

• Firefox gives "javascript error"
• Chrome gives "script timeout"
• Safari gives "no such frame"

As far as I can tell, we should fall into:

If promise is still pending and timer's timeout fired flag is set, return error with error code script timeout.

…because the setTimeout call never calls its argument.

And thus the command should always take the current script timeout to complete. (It doesn't in any browser.)

This seems wrong, and we should move towards interop here.

[whimboo]

This is a dupe of #1708, right?