Put data URL dedicated workers in their own agent cluster

Tests:

* There should be a test for this where you create a data URL worker and try to message it SAB and WebAssembly.Module and check that it results in messageerror events.
* web-platform-tests/wpt#21146 and web-platform-tests/wpt#22928 test that inside a data URL worker you can share memory with a further nested data URL worker that is same-origin with the opaque origin of the data URL worker.

Fixes #5254.

Author: annevk

source

@@ -96580,13 +96580,13 @@ new PaymentRequest(…); // Allowed to use
 
   <p>To <dfn data-x="obtaining a worker/worklet agent">obtain a worker/worklet agent</dfn>, given an
   <span>environment settings object</span> or null <var>outside settings</var>, a boolean
-  <var>isTopLevel</var>, and a boolean <var>canBlock</var>, run these steps:</p>
+  <var>createsCluster</var>, and a boolean <var>canBlock</var>, run these steps:</p>
 
   <ol>
    <li><p>Let <var>agentCluster</var> be null.
 
    <li>
-    <p>If <var>isTopLevel</var> is true, then:</p>
+    <p>If <var>createsCluster</var> is true, then:</p>
 
     <ol>
      <li><p>Set <var>agentCluster</var> to a new <span>agent cluster</span>.</p></li>
@@ -96625,9 +96625,9 @@ new PaymentRequest(&hellip;); // Allowed to use
   </ol>
 
   <p>To <dfn>obtain a dedicated/shared worker agent</dfn>, given an <span>environment settings
-  object</span> <var>outside settings</var> and a boolean <var>isShared</var>, return the result of
-  <span>obtaining a worker/worklet agent</span> given <var>outside settings</var>,
-  <var>isShared</var>, and true.</p>
+  object</span> <var>outside settings</var> and a boolean <var>createsCluster</var>, return the
+  result of <span>obtaining a worker/worklet agent</span> given <var>outside settings</var>,
+  <var>createsCluster</var>, and true.</p>
 
   <p>To <dfn>obtain a worklet agent</dfn>, given an <span>environment settings object</span>
   <var>outside settings</var>, return the result of <span>obtaining a worker/worklet agent</span>
@@ -109194,7 +109194,18 @@ interface <dfn interface>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope
 
   <ol>
    <li><p>Let <var>is shared</var> be true if <var>worker</var> is a <code>SharedWorker</code>
-   object, and false otherwise.</p></li>
+   object; otherwise false.</p></li>
+
+   <li>
+    <p>Let <var>createsCluster</var> be true if <var>is shared</var> is true or <var>url</var>'s
+    <span data-x="concept-url-scheme">scheme</span> is "<code data-x="">data</code>"; otherwise
+    false.</p>
+
+    <p class="note"><var>url</var> can be used here, because a redirect to a <span data-x="data
+    protocol"><code>data:</code> URL</span> results in a network error.</p>
+    <!-- Nevertheless we probably ought to move this in due course to support sandboxing:
+         https://github.com/w3c/webappsec-csp/issues/279 -->
+   </li>
 
    <li><p>Let <var>owner</var> be the <span>relevant owner to add</span> given <var>outside
    settings</var>.</p></li>
@@ -109210,7 +109221,7 @@ interface <dfn interface>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope
 
    <li><p>Let <var>agent</var> be the result of <span data-x="obtain a dedicated/shared worker
    agent">obtaining a dedicated/shared worker agent</span> given <var>outside settings</var> and
-   <var>is shared</var>. Run the rest of these steps in that agent.</p></li>
+   <var>createsCluster</var>. Run the rest of these steps in that agent.</p></li>
 
    <li>
     <p>Let <var>realm execution context</var> be the result of <span>creating a new realm</span>