Improve navigate for service worker hooks

Before this patch, we didn't have a concept that holds reserved
client's information referenced from a service worker during a
navigation (more precisely before the actual global object and its
relevant environment settings object for the resulting resource is
created).

This patch defines an "environment" concept (a supertype object of an
environment settings object) that has its id, creation URL and target
browsing context such that service workers' FetchEvent hanlder can
reference this client information before the corresponding environmet
setings object is actually created.

The changes include:
 - Create an environment (a reserved client) during a navigation
 - Set request's reserved client id such that fetch (and Handle Fetch)
 can use it
 - Set request's target client id such that fetch (and Handle Fetch)
 can use it
 - Add Window object's active worker internal slot
 - Add WorkerGlobalObject's active worker internal slot
 - Add "match service worker registration" logic to attach controller

Related issue: w3c/ServiceWorker#870
Call flow: w3c/ServiceWorker#870 (comment)
Expected behavior: w3c/ServiceWorker#870 (comment)
Related changes:
 - SW (WIP): w3c/ServiceWorker@df39d89
 - Fetch: whatwg/fetch#383