[System Extensions] Setup testing framework for testing window management API.

Due to limitations in the current platform, we test system extension APIs via enabling them using a flag.
Then, tests can then be written as js code and passed to the helper to
run inside a service worker accessing the API methods on a sample page.

Bug: b/221124035
Change-Id: If1c5157d029a096806be0c43ab39adaaecd8f7dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3521113
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Commit-Queue: Clement Yan <clamclamyan@google.com>
Cr-Commit-Position: refs/heads/main@{#984138}