Only allows instrument.set() in active service worker

Like other service worker family features, we should only allow
instrument.set() in active service worker.

This fixes w3c#223 issue.

Author: romandev

index.html

@@ -494,6 +494,33 @@ <h2>
             When called, this method executes the following steps:
           </p>
           <ol>
+            <li>Let <var>registration</var> be the <a>PaymentInstrument</a>'s
+            associated <a>service worker registration</a>.
+            </li>
+            <li>If <var>registration</var> has no <a>active worker</a>, run the
+            following substeps:
+              <ol>
+                <li>If <var>registration</var> has no <a>installing worker</a>
+                and no <a>waiting worker</a>, then reject a <a>Promise</a> with
+                a <a>DOMException</a> whose name is
+                "<code><a>InvalidStateError</a></code>" and terminate these
+                steps.
+                </li>
+                <li>Wait for the <a>installing worker</a> or <a>waiting
+                worker</a> of <var>registration</var> to become its <a>active
+                worker</a>.
+                </li>
+                <li>If <var>registration</var> fails to activate either worker,
+                reject <var>promise</var> with a
+                <code><a>DOMException</a></code> whose name is
+                "<code><a>InvalidStateError</a></code>" and terminate these
+                steps.
+                </li>
+                <li>Once <var>registration</var> has an <a>active worker</a>,
+                proceed with the steps below.
+                </li>
+              </ol>
+            </li>
             <li>Let <var>permission</var> be the result of running
               <a data-cite="!permissions#dfn-retrieve-the-permission-state">retrieving
               the permission state</a> of the permission associated with
@@ -1993,8 +2020,10 @@ <h2>
         <dd>
           The terms <dfn data-cite=
           "!SERVICE-WORKERS#service-worker-concept">service worker</dfn>,
-          <dfn data-cite="!SERVICE-WORKERS#dfn-service-worker-client">service
-          worker client</dfn>, <code><dfn data-cite=
+          <dfn>service worker registration</dfn>, <dfn>active worker</dfn>,
+          <dfn>installing worker</dfn>, <dfn data-cite=
+          "!SERVICE-WORKERS#dfn-service-worker-client">service worker
+          client</dfn>, <code><dfn data-cite=
           "!SERVICE-WORKERS#service-worker-registration-concept">ServiceWorkerRegistration</dfn></code>,
           <code><dfn data-cite=
           "!SERVICE-WORKERS#service-worker-global-scope">ServiceWorkerGlobalScope</dfn></code>,