Editorial: formalize the concept of a realm's agent

Follows tc39/ecma262#3058. See tc39/ecma262#1357 for more background.

source

@@ -97228,11 +97228,14 @@ new PaymentRequest(…); // Allowed to use
    <li><p>Return <var>agent</var>.</p></li>
   </ol>
 
+  <p>For a <span>realm</span> <var>realm</var>, the <span>agent</span> whose [[Signifier]] is
+  <var>realm</var>.[[AgentSignifier]] is <dfn data-x="realm's agent" export data-lt="agent"
+  for="realm">the realm's agent</dfn>.</p>
+
   <p>The <dfn export>relevant agent</dfn> for a <span>platform object</span>
   <var>platformObject</var> is <var>platformObject</var>'s <span
-  data-x="concept-relevant-realm">relevant realm</span>'s <span>agent</span>. <span class="XXX"
-  data-x="">This pointer is not yet defined in the JavaScript specification; see <a
-  href="https://github.com/tc39/ecma262/issues/1357">tc39/ecma262#1357</a>.</span></p>
+  data-x="concept-relevant-realm">relevant realm</span>'s <span data-x="realm's
+  agent">agent</span>.</p>
 
   <p class="note">The agent equivalent of the <span>current realm</span> is the <span>surrounding
   agent</span>.</p>
@@ -97380,7 +97383,7 @@ new PaymentRequest(&hellip;); // Allowed to use
      <li><p><span>Assert</span>: <var>outside settings</var> is not null.</p></li>
 
      <li><p>Let <var>ownerAgent</var> be <var>outside settings</var>'s <span data-x="environment
-     settings object's realm">realm</span>'s agent.</p></li>
+     settings object's realm">realm</span>'s <span data-x="realm's agent">agent</span>.</p></li>
 
      <li><p>Set <var>agentCluster</var> to the agent cluster which contains
      <var>ownerAgent</var>.</p></li>
@@ -97683,10 +97686,6 @@ new PaymentRequest(&hellip;); // Allowed to use
 
    <li><p>Let <var>realm</var> be <var>realm execution context</var>'s Realm component.</p></li>
 
-   <li><p>Set <var>realm</var>'s agent to <var>agent</var>. <span class="XXX" data-x="">This pointer
-   is not yet defined in the JavaScript specification; see <a
-   href="https://github.com/tc39/ecma262/issues/1357">tc39/ecma262#1357</a>.</span></p></li>
-
    <li>
     <p>If <var>agent</var>'s <span>agent cluster</span>'s <span
     data-x="agent-cluster-cross-origin-isolation">cross-origin isolation mode</span> is "<code