Add request's reserved client id and target client id

This patch defines request's reserved client id and target client id
that are set to a reserved environment's id and the target browsing
context's active document's settings object's id respectively during a
navigation. The given values are primarily used to provide the
reserved client's id and target client's id to service workers' fetch
event handlers.

Related issue: w3c/ServiceWorker#870
Call flow: w3c/ServiceWorker#870 (comment)
Expected behavior: w3c/ServiceWorker#870 (comment)
Related changes:
 - HTML: whatwg/html#1776
 - SW (WIP): w3c/ServiceWorker@df39d89

Overview.src.html

@@ -661,6 +661,14 @@ <h4>Requests</h4>
 <dfn title=concept-request-client data-export data-dfn-for=request>client</dfn> (null or an
 <span data-anolis-spec=html>environment settings object</span>).
 
+<p>A <span title=concept-request>request</span> has an associated
+<dfn title=concept-request-reserved-client-id data-export data-dfn-for=request>reserved client
+id</dfn> (null or a string). Unless stated otherwise it is null.
+
+<p>A <span title=concept-request>request</span> has an associated
+<dfn title=concept-request-target-client-id data-export data-dfn-for=request>target client id</dfn>
+(null or a string). Unless stated otherwise it is null.
+
 <p>A <span title=concept-request>request</span> has an associated
 <dfn title=concept-request-window data-export data-dfn-for=request>window</dfn>
 ("<code>no-window</code>", "<code>client</code>", or an