[spec] Normative: Prohibit IndexedDB serialization of Modules

Based on discussions in the WebAssembly CG, it sounds like IndexedDB
serialization of WebAssembly Modules has the following problems:
- On some engines, it's impractical to serialize more than just the
  binary module data
- On some engines, upgrading from one version to another would cause
  the serialized module to be invalidated, with complicated implementation
  logic and a loss of reliability
- Some engines plan to just return an error for IndexedDB module
  serialization as a design decision

The purpose of IndexedDB Module serialization is for reliable performance,
and these items provide risks to that. This patch proposes that, initially,
IndexedDB serialization would not be provided for WebAssembly modules.

document/web-api/index.bs

@@ -141,13 +141,12 @@ Web user agents must augment the {{Module}} interface with the <code>[<a extende
 
 The [=serialization steps=], given |value|, |serialized|, and |forStorage|, are:
 
-    1. If |forStorage| is true and the [=relevant settings object=]
-        of |value| is not [=contextually secure=], throw a
+    1. If |forStorage| is true, throw a
         "<a exception>DataCloneError</a>" {{DOMException}}
 
     1. Set |serialized|.\[[Bytes]] to the [=sub-serialization=] of |value|.\[[Bytes]].
 
-    1. If |forStorage| is *false*, set |serialized|.\[[AgentCluster]] to the
+    1. Set |serialized|.\[[AgentCluster]] to the
        [=current Realm=]'s corresponding [=agent cluster=].
 
 The [=deserialization steps=], given |serialized| and |value|, are:
@@ -156,8 +155,7 @@ The [=deserialization steps=], given |serialized| and |value|, are:
 
     1. Set |value|.\[[Bytes]] to |bytes|.
 
-    1. If |serialized| has an \[[AgentCluster]] field, and if |targetRealm|'s
-        corresponding [=agent cluster=] is not
+    1. If |targetRealm|'s corresponding [=agent cluster=] is not
         |serialized|.\[[AgentCluster]], then throw a
         "<a exception>DataCloneError</a>" {{DOMException}}.
 
@@ -173,10 +171,6 @@ Given the above engine optimizations, structured serialization provides develope
 explicit control over both compiled-code caching and cross-window/worker code
 sharing.
 
-Note: The web platform does not permit cross-origin sharing of serialized Modules: in the
-case of |forStorage| being true, IndexedDB separates storage per origin; in the case of
-|forStorage| being false, agent clusters in HTML do not span multiple origins.
-
 <h2 id="conventions">Developer-Facing Display Conventions</h2>
 
 <em>This section is non-normative.</em>