Support test_driver.set_permission() inside .any.js (for workers)

[saschanaz]

test_driver is only designed to work on the window context. Which is fine as many webdriver commands like moving pointers only makes sense there, but set_permission is often required also for workers to enable certain features there, e.g. service worker-specific showNotification() behind notification permission.

Perhaps we can:

• Provide some way to tunnel the test_driver requests to the main thread as service_worker_test() does?
• Or provide a way to run a script on the main window before starting the tests in the worker context?