(WIP) Improve registration matching and exposing reserved/target client

This patch fixes the client matching flow and exposes reserved client
and target client information by adding necessary hooks to fetch and
HTML.

Related issue: #870
Call flow: #870 (comment)
Expected behavior: #870 (comment)
Related changes:
 - HTML: whatwg/html#1776
 - Fetch: whatwg/fetch#383

* Work on Handle Fetch is done. Work on the client API is in progress.