Add Client's reserved state

Once a Client object is created, the value client.reserved returns
shouldn't change. This defines Client's reserved state which Create
Client algorithm initializes when a Client object is created.

Fixes #1003

docs/index.bs

@@ -999,6 +999,8 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
 
     <p>A {{Client}} object has an associated <dfn for="Client">service worker client</dfn> (a <a href="#dfn-service-worker-client">service worker client</a>).</p>
 
+    <p>A {{Client}} object has an associated <dfn for="Client">reserved state</dfn>, which is either true or false.</p>
+
     <p>A {{WindowClient}} object has an associated <dfn id="dfn-service-worker-client-visibilitystate">visibility state</dfn>, which is one of {{Document/visibilityState}} attribute value.</p>
 
     <p>A {{WindowClient}} object has an associated <dfn id="dfn-service-worker-client-focusstate">focus state</dfn>, which is either true or false (initially false).</p>
@@ -1020,7 +1022,7 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
     <section>
       <h4 id="client-reserved">{{Client/reserved}}</h4>
 
-      <p>The <dfn attribute for="Client"><code>reserved</code></dfn> attribute <em class="rfc2119" title="MUST">must</em> return true if the <a>context object</a>'s associated <a for="Client">service worker client</a>'s <a for="environment">execution ready flag</a> is unset, and false otherwise.</p>
+      <p>The <dfn attribute for="Client"><code>reserved</code></dfn> attribute <em class="rfc2119" title="MUST">must</em> return the <a>context object</a>'s associated <a for="Client">reserved state</a>.</p>
 
       <p class="issue">Defining the execution ready flag is a work in progress. See <a href="https://github.com/whatwg/html/pull/2004">the pull request to HTML</a>.</p>
     </section>
@@ -4011,6 +4013,7 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
     <ol>
       <li>Let <var>clientObject</var> be a new {{Client}} object.</li>
       <li>Set <var>clientObject</var>'s <a for="Client">service worker client</a> to <var>client</var>.</li>
+      <li>Set <var>clientObject</var>'s <a for="Client">reserved state</a> to true if <var>client</var>'s <a for="environment">execution ready flag</a> is unset, and false otherwise.</li>
       <li>Return <var>clientObject</var>.</li>
     </ol>
   </section>