[worklets] Update agent and realm allocation

Follows whatwg/html#5411.

Author: domenic

worklets/Overview.bs

@@ -219,17 +219,17 @@ Note:
 When a user agent is to <dfn export>create a WorkletGlobalScope</dfn>, given |workletGlobalScopeType|,
 |moduleResponsesMap|, and |outsideSettings|, it <em>must</em> run the following steps:
 
-    1. Create the <a>worklet global scope execution environment</a> and run the rest of these steps
-        in that context.
+    1. Let |agent| be the result of [=obtain a worklet agent|obtaining a worklet agent=] given
+        |outsideSettings|. This agent corresponds to the a <a>worklet global scope execution
+        environment</a>. Run the rest of these steps in that context.
 
-    2. Call the JavaScript <a>InitializeHostDefinedRealm</a> abstract operation with the following
-        customizations:
+    2. Let |realmExecutionContext| be the result of
+        [=create a new JavaScript realm|creating a new JavaScript realm=] given <var>agent</var> and
+        the following customizations:
 
         - For the global object, create a new |workletGlobalScopeType| object. Let
             |workletGlobalScope| be the created object.
 
-        - Let |realmExecutionContext| be the created JavaScript execution context.
-
     3. Let |insideSettings| be the result of <a>set up a worklet environment settings object</a>
         given |realmExecutionContext|, and |outsideSettings|.