Define client.reserved

This patch adds client.reserved attribute to Client class.
client.reserved reflects the state of its associtated environment's
execution ready flag. When an environment is initially created (not
fully ready with the environment settings object/global object/fetched
main resource), this boolean attribute returns true. When the
environment's execution ready flag is set (the environment settings
object is created and the main resource is fetched), client.reserved
returns false.

docs/index.bs

@@ -1056,6 +1056,7 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
         readonly attribute USVString url;
         readonly attribute FrameType frameType;
         readonly attribute DOMString id;
+        readonly attribute boolean reserved;
         void postMessage(any message, optional sequence<object> transfer = []);
       };
 
@@ -1075,7 +1076,7 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
       };
     </pre>
 
-    <p>A <code><dfn interface id="client-interface">Client</dfn></code> object has an associated <dfn id="dfn-service-worker-client-client" for=Client>service worker client</dfn> (a <a href="#dfn-service-worker-client">service worker client</a>).</p>
+    <p>A <code><dfn interface id="client-interface">Client</dfn></code> object has an associated <dfn id="dfn-service-worker-client-client" for="Client">service worker client</dfn> (a <a href="#dfn-service-worker-client">service worker client</a>).</p>
 
     <p>A <code><dfn interface id="window-client-interface">WindowClient</dfn></code> object has an associated <dfn id="dfn-service-worker-client-visibilitystate">visibility state</dfn>, which is one of {{Document/visibilityState}} attribute value.</p>
 
@@ -1116,6 +1117,12 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
       <p>The <dfn attribute for="Client" id="client-id-attribute"><code>id</code></dfn> attribute <em class="rfc2119" title="MUST">must</em> return its associated <a href="#dfn-service-worker-client-client">service worker client</a>'s <a for="environment">id</a>.</p>
     </section>
 
+    <section>
+      <h4 id="client-reserved">{{Client/reserved}}</h4>
+
+      <p>The <dfn attribute for="Client" id="client-reserved-attribute"><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>execution ready flag</a> is unset, and false otherwise.</p>
+    </section>
+
     <section algorithm="client-postmessage">
       <h4 id="client-postmessage">{{Client/postMessage(message, transfer)}}</h4>