Ensure that all popups get closed for popup-redirect-cache.https.html

In my earlier refactor of popup-redirect-cache.https.html I
overlooked that the one BroadcastChannel "close" message was
being used to clean up two windows for each test... To get the
same functionality using the dispatcher, we need to add two
close messages to the queue instead of one. This changes makes
the two parts of the test identical, allowing us to remove
some duplicated code.

Also, I'm hoping this fixes some WPT timeouts in Firefox
related to the previous CL:
https://github.com/web-platform-tests/wpt/pull/35570

Bug: 1354602
Change-Id: Ie8b883b7003cd233747ba4fe6e0f2508ee057144
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3855346


Reviewed-by: Arthur Hemery <ahemery@chromium.org>
Commit-Queue: Andrew Williams <awillia@google.com>
Cr-Commit-Position: refs/heads/main@{#1039267}