Use environment, reserved client, and target client id
This patch fixes the steps in Handle Fetch that
- Get various clients' info and expose them to fetch event handlers
- Match/attach a controller, if available, to the given environment
by referencing the request's reserved client and target client id.
Related commits:
- HTML: https://github.com/whatwg/html/commit/2b93f9ec35b152e58d3e181bea8c45d789bac949
- Fetch: https://github.com/whatwg/fetch/commit/fb87b70df0af3a5e6bc7bb9a91b4dc7eb4924acb
NOTE: Changing the methods of Clients/Client will be done as a
separate patch.
