Raise the bar for SharedArrayBuffer via postMessage()

This depends on the work to add Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy. It specifies how postMessage() is affected when both those headers are set for the agent clusters they impact.

Closes #4732.

source

@@ -8275,13 +8275,25 @@ interface <dfn>DOMStringList</dfn> {
       <p>If ! <span>IsSharedArrayBuffer</span>(<var>value</var>) is true, then:
 
       <ol>
+       <li><p>Let <var>agentCluster</var> be the <span>current Realm Record</span>'s corresponding
+       <span>agent cluster</span>.</p></li>
+       <!-- Should this use the surrounding agent's agent cluster instead? -->
+
+       <li>
+        <p>If <var>agentCluster</var>'s <span>allow sidechannel attacks</span> is set to false, then
+        throw a <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p>
+
+        <p class="note">This check is only needed once as <span>allow sidechannel attacks</span>
+        cannot change over time and a <code>SharedArrayBuffer</code> cannot leave an <span>agent
+        cluster</span>.</p>
+       </li>
+
        <li><p>If <var>forStorage</var> is true, then throw a
        <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
 
        <li><p>Set <var>serialized</var> to { [[Type]]: "SharedArrayBuffer", [[ArrayBufferData]]:
        <var>value</var>.[[ArrayBufferData]], [[ArrayBufferByteLength]]: <var>size</var>,
-       [[AgentCluster]]: the <span>current Realm Record</span>'s corresponding <span>agent
-       cluster</span> }.</p></li>
+       [[AgentCluster]]: <var>agentCluster</var> }.</p></li>
       </ol>
      </li>