Update browsingContext.UserPromptOpened event parameters (#738)

* Update browsingContext.UserPromptOpened event parameters

Modifies the `handler` property of the event parameters to be
of type `session.UserPromptHandlerType`.

---------

Co-authored-by: Henrik Skupin <[email protected]>

Author: jimevans

index.bs

@@ -1667,8 +1667,6 @@ session.SystemProxyConfiguration = (
 [=Remote end definition=] and [=local end definition=]
 
 <pre class="cddl remote-cddl local-cddl">
-session.UserPromptHandlerType = "accept" / "dismiss" / "ignore";
-
 session.UserPromptHandler = {
   ? alert: session.UserPromptHandlerType,
   ? beforeUnload: session.UserPromptHandlerType,
@@ -1681,6 +1679,17 @@ session.UserPromptHandler = {
 The <code>session.UserPromptHandler</code> type represents the configuration of
 the user prompt handler.
 
+#### The session.UserPromptHandlerType Type #### {#type-session-UserPromptHandlerType}
+
+[=Remote end definition=] and [=local end definition=]
+
+<pre class="cddl remote-cddl local-cddl">
+session.UserPromptHandlerType = "accept" / "dismiss" / "ignore";
+</pre>
+
+The <code>session.UserPromptHandlerType</code> type represents the behavior
+of the user prompt handler.
+
 #### The session.SubscriptionRequest Type #### {#type-session-SubscriptionRequest}
 
 <pre class="cddl remote-cddl">
@@ -4623,7 +4632,7 @@ closed</dfn> steps given |window|, |type|, |accepted| and optional |user text|
 
         browsingContext.UserPromptOpenedParameters = {
           context: browsingContext.BrowsingContext,
-          handler: "accept" / "dismiss" / "ignore",
+          handler: session.UserPromptHandlerType,
           message: text,
           type: browsingContext.UserPromptType,
           ? defaultValue: text