Web API: adopt [Serializable=SameAgentCluster]

As introduced by whatwg/html#4940.

Author: annevk

document/web-api/index.bs

@@ -139,24 +139,20 @@ Note: This algorithm accepts a {{Response}} object, or a
 
 <h2 id="serialization">Serialization</h2>
 
-Web user agents must augment the {{Module}} interface with the <code>[<a extended-attribute>Serializable</a>]</code> extended attribute.
+Web user agents must augment the {{Module}} interface with the <code>[<a extended-attribute>Serializable</a>]</code> extended attribute set to <code>SameAgentCluster</code>.
 
 The [=serialization steps=], given |value|, |serialized|, and |forStorage|, are:
 
     1. If |forStorage| is true, throw a "<a exception>DataCloneError</a>" {{DOMException}}
 
     1. Set |serialized|.\[[Bytes]] to the [=sub-serialization=] of |value|.\[[Bytes]].
 
-    1. Set |serialized|.\[[AgentCluster]] to the [=current Realm=]'s corresponding [=agent cluster=].
-
 The [=deserialization steps=], given |serialized| and |value|, are:
 
     1. Let |bytes| be the [=sub-deserialization=] of |serialized|.\[[Bytes]].
 
     1. Set |value|.\[[Bytes]] to |bytes|.
 
-    1. If |targetRealm|'s corresponding [=agent cluster=] is not |serialized|.\[[AgentCluster]], then throw a "<a exception>DataCloneError</a>" {{DOMException}}.
-
     1. [=Compile a WebAssembly module=] from |bytes| and set |value|.\[[Module]] to the result.
 
 Engines should attempt to share/reuse internal compiled code when performing