Behavior of window.close()

[annevk]

I talked with @bzbarsky about window.close() a bit because of web-platform-tests/wpt#15518. It seems window.close() does two things:

1. Immediately sets a flag that immediately affects window.closed and named targeting. (Chrome seems similar for window.closed; Safari not (branches on "discard"); did not test named targeting.)
2. Queues a task to maybe discard. (There's no task involved currently, but there is in all browsers.)

This does raise the question of what kind of things can happen between window.close() and discarding. E.g., can you navigate, can you go back in history? More tests are needed.

Related PR that needs changes because of this: #4402.

Somewhat related: #1866, #4363.

[rniwa]

@cdumez

[annevk]

web-platform-tests/wpt#15649 has tests. I also pushed a fix to #4402.