Considering reducing the scope of the CookieStore API to match Firefox and Safari implementations

[bakulf]

Safari and Firefox agreed on a subset of the CookieStore API to match only the document.cookie behavior. The subset is the following:

1. Only cookie names and values are shared via the CookieStore.get()/getAll() methods. This way, we do not expose more than what document.cookie already offers. See When getting cookies only expose their name and value #238.
2. We don't implement the CookieStoreManager and the ServiceWorkerGlobalScope.oncookiechange event handler with its ExtendableCookieChangeEvent event because we don't want to spawn ServiceWorkers at any cookie change
3. To follow up on the previous point, the CookieStore.onchange event handler is also exposed to ServiceWorkers.

This is currently implemented in Firefox and enabled in Nightly.

CC @annevk

[smaug----]

@inexorabletash